Orthopedic joint distraction device

ABSTRACT

An orthopedic distraction device is provides. The orthopedic distraction device includes a first upper paddle for engaging a first bone of a joint, a lower paddle for engaging a second bone of the joint and a displacement mechanism. The displacement mechanism includes a drive assembly operable to move the upper paddle relative to the lower paddle. The lower paddle is releasably connected to the displacement mechanism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/309,711 filed Mar. 17, 2016; 62/300,597 filed Feb. 26, 2016;62/218,840 filed Sep. 15, 2015; and 62/137,615 filed Mar. 24, 2015, theentire disclosures of which are incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION

Joint replacement surgery is performed on patients with degenerativejoint diseases, such osteoarthritis and arthrosis, with the goals ofrelieving pain and restoring function, thus improving the quality oflife for the patient. Although joint replacement surgery is exceedinglycommon, with approximately 700,000 knee replacement procedures performedannually in the U.S., it has been reported that a significant portion ofpatients (approximately one in five) are not satisfied with the resultsof their surgery. While this may be due to a number of factors, such aspatient expectations, it is suspected that surgical technique relatedfactors may play an important role in the number of cases that have lessthan optimal outcomes. In fact, several clinical studies have indicatedthat soft tissue related factors, such as instability and stiffness, arethe leading cause for failure of total knee arthroplasty (TKA).

The act of achieving the appropriate soft-tissue tension and balance injoint replacement surgery is still regarded as somewhat of an art formby surgeons. This is partly because the act of assessing the tension inthe soft tissues that surround a joint is largely a subjective processwhere the surgeon manually applies forces and moments to one side of thejoint and observes the opening or compliance of the joint under theapplied force by feel and by eye. Thus the assessment of soft tissuetension may vary depending on the surgeon performing the assessment, howthey were trained, hold the limb by hand, and this may also vary fromday to day, or from their left to right hand.

The standard of care in joint replacement surgery today is to use manualinstrumentation which includes alignment rods, cutting blocks,provisional trial implants, and tensioning tools such as laminarspreaders or specifically designed manual spreaders. Robot andcomputer-assisted surgery systems have been introduced in the late 90'sand have been increasing in development and use. However, most ofsystems currently on the market only partially address the soft tissuetensioning and balancing problem. Moreover, these systems still requirea large number of instruments and provisional trial components to beavailable in the operating room.

BRIEF SUMMARY OF THE INVENTION

In accordance with a preferred embodiment, the present inventionprovides an orthopedic distraction device comprising a first upperpaddle for engaging a first bone of a joint, a lower paddle for engaginga second bone of the joint, and a displacement mechanism having a driveassembly operable to move the upper paddle relative to the lower paddle.The lower paddle is releasably connected to the displacement mechanism.

The orthopedic distraction device further includes a second upper paddlefor engaging the first bone of the joint. The first upper paddle extendsfurther from the displacement mechanism than the second upper paddle.One of the first and second upper paddles includes an inwardly extendingrelief for clearance. The lower paddle includes fasteners for fasteningto the second bone. The fastener is at least one of a pin, a plugfastener, and a screw. The lower paddle also includes at least one of akeel opening for receiving a keel punch, a fastener opening forreceiving a fastener, and guide members for receiving a keel punch. Thelower paddle is also sized and shaped to match a size and shape of animplant to be implanted in the second bone.

In accordance with another preferred embodiment, the present inventionprovides an orthopedic distraction device comprising an upper paddle, aplurality of augments each releasably connectable to the upper paddlefor engaging a first bone of a joint, a lower paddle for engaging asecond bone of the joint, and a displacement mechanism having a driveassembly operable to move one of the upper and lower paddles relative tothe other of the upper and lower paddles. The orthopedic distractiondevice further includes another upper paddle. Each of the plurality ofaugments is configured to articulate with the first bone or a femoraltrial implant, and includes a concave upper surface. Each of theplurality of augments when connected to the upper paddles includes alongitudinal axis extending at a non-perpendicular and non-parallelangle relative to a coronal plane of the displacement mechanism.

The upper paddle includes contact surfaces for articulating with afemur, and when the augment is connected to the upper paddle the contactsurfaces are below the augment.

In accordance with a preferred embodiment, the present inventionprovides an orthopedic distraction device comprising at least one of amedial upper paddle and a lateral upper paddle for engaging a first boneof a joint, a lower paddle for engaging a second bone of the joint, anda displacement mechanism having a hermetically sealed drive assemblyoperable to displace the at least one of a medial upper paddle and alateral upper paddle relative to the lower paddle.

The orthopedic distraction device further includes the other of the atleast one of a medial upper paddle and a lateral upper paddle and acontroller. The controller is configured to apply a first displacementforce to the medial upper paddle and a second displacement forcediffering from the first displacement force to the lateral upper paddle.The drive assembly includes a drive mechanism operably connected to theat least one of a medial upper paddle and a lateral upper paddle. Thedrive mechanism includes a plunger driven by a motor for moving the atleast one of a medial upper paddle and a lateral upper paddle. The drivemechanism is preferably a spindle drive.

The displacement mechanism comprises a paddle connector connectable tothe at least one of a medial upper paddle and a lateral upper paddle,and a drive mechanism, and a sensor positioned below the drive assemblyfor measuring a force applied to the at least one of a medial upperpaddle and a lateral upper paddle. The paddle connector moves relativeto the drive mechanism.

The displacement mechanism further comprises a bellow for hermeticallyenclosing the paddle connector, and a flexure bracket for supporting thedrive assembly.

The displacement mechanism also includes a housing body, and a flexurebracket connected to the housing body. The flexure bracket secures thedrive assembly within the housing body.

The orthopedic distraction device further includes a controlleroperatively in communication with the displacement mechanism, andconfigured to move the displacement mechanism to receive a predeterminedload force. The displacement mechanism further includes a sensoroperatively in communication with the controller for measuring the loadforce applied to the at least one of a medial upper paddle and a lateralupper paddle.

The controller is configured to apply a displacement force to displacethe at least one of a medial upper paddle and a lateral upper paddlerelative to the lower paddle when engaging the first and second bones ofthe joint. The controller is also configured to vary the displacementforce based on flexion angle of the first and second bones of the joint,and configured to determine a gap spacing between the at least one of amedial upper paddle and a lateral upper paddle, and the lower paddlebased on the displacement force and a deflection factor.

In accordance with another preferred embodiment, the present inventionprovides an orthopedic distraction device comprising an upper paddle forengaging a first bone of a joint, a lower paddle for engaging a secondbone of the joint, a displacement mechanism operable to displace theupper paddle relative to the lower paddle, and a controller operativelyin communication with the displacement mechanism, and configured toapply varying displacement forces to displace the upper paddle from thelower paddle based on a relative position between the first and secondbones of the joint. The displacement mechanism can include a driveassembly operable to displace the upper paddle relative to the lowerpaddle.

The displacement mechanism includes a drive assembly to displace theupper paddle relative to the low paddle.

The controller is configured to apply varying displacement forcesthroughout a range of motion of the joint, and apply varyingdisplacement forces based on a joint angle of the joint. Further, thecontroller includes a memory having stored thereon a predetermined forceprofile for applying said varying displacement forces throughout a rangeof motion of the joint. Furthermore, the controller includes apredefined force versus flexion angle profile stored in a memory fordetermining the varying displacement forces to apply.

The force versus flexion angle profile is defined by a user and storedin a memory of the controller for determining the varying displacementforces to apply. The force versus flexion angle profile for determiningthe varying displacement forces to apply is adjustable by a userintraoperatively during surgery. The varying displacement forces areadjustable. The force versus flexion angle profile for determining thevarying displacement forces to apply is adjustable by a user throughouta range of motion of the joint. The force versus flexion angle profileis displayed on a display. Further, the force versus flexion angleprofile on the display is adjustable by a user. Furthermore, the forceversus flexion angle profile on the display includes node control pointsadjustable by a user.

The controller is also configured to measure a gap spacing between thefirst and second bones of the joint upon applying said varyingdisplacement forces and determine an implant position based off themeasured gap spacing.

In accordance with a preferred embodiment, the present inventionprovides an orthopedic distraction device comprising a first upperpaddle for engaging a first bone of a joint, a lower paddle for engaginga second bone of the joint, and a displacement mechanism. Thedisplacement mechanism includes a housing, and a drive assembly withinthe housing operable to displace the first upper paddle relative to thelower paddle. The drive assembly is axially movable between a firstposition and a second position spaced from the first position.

The orthopedic distraction device further includes a second upper paddlefor engaging the first bone, and a flexure bracket mounted to thehousing with the drive assembly is mounted to the flexure bracket. Theflexure bracket includes a rigid portion and a flexure portion moveablerelative to the rigid portion.

The orthopedic distraction device further includes a sensor positionedwithin the housing and below the drive assembly. The drive assemblyengages the sensor in both the first and second positions, and isconnected to the first upper paddle.

A bellows assembly is connected to the first upper paddle and driveassembly. The bellows assembly is movable relative to the driveassembly.

In accordance with another preferred embodiment, the present inventionprovides an orthopedic instrument kit. The kit includes a plurality offemoral trail implants of incrementally different sizes and anorthopedic distraction device. The orthopedic distraction deviceincludes a first upper paddle, a plurality of lower paddles, and adisplacement mechanism having a drive assembly operable to move theupper paddle relative to the lower paddle, wherein each lower paddle isindependently connectable to the displacement mechanism.

The kit further includes a plurality of tibial implants. Each of theplurality of lower paddles has an overall profile sized and shaped tocorrespond to a size and shape of an overall profile of the plurality oftibial implants. Further, the kit includes a plurality of augments eachreleasably connectable to the first upper paddle. Each of the pluralityof augments has an articulating surface that corresponds in size to asize of each of the plurality of femoral trial implants.

In accordance with a preferred embodiment, the present inventionprovides an orthopedic distraction device with a controller and adisplay. The orthopedic distraction device includes medial and lateralupper paddles for engaging a first bone of a joint, a lower paddle forengaging a second bone of the joint, and a displacement mechanism havinga drive assembly operable to supply a displacement force to the upperand lower paddles. The orthopedic distraction device further includes anon-transitory computer-readable medium including instructions that,when executed by a processor, cause the processor to measure adisplacement between the upper and lower paddles, and display on adisplay the displacement forces applied to the upper and lower paddlesverses displacement.

In accordance with another preferred embodiment, the present inventionprovides a computer aided orthopedic surgery system that includes athree dimensional position tracking system and an orthopedic distractiondevice. The orthopedic distraction device includes upper paddles forengaging a first bone of a joint, and a lower paddle for engaging asecond bone of the joint. The lower paddle includes a reference markertrackable by the three dimensional position tracking system. Theorthopedic distraction device further includes a displacement mechanismhaving a drive assembly operable to move the upper paddles relative tothe lower paddle. The computer aided orthopedic surgery system furtherincludes a computer having a memory for tracking the reference markerand a display for displaying the tracked reference marker. Furthermore,the computer aided orthopedic surgery system can include a roboticsystem having a robotic arm attached to the orthopedic distractiondevice.

In accordance with a preferred embodiment, the present inventionprovides a computer aided orthopedic surgery system that includes athree dimensional position tracking system and an orthopedic distractiondevice. The orthopedic distraction device includes upper paddles forengaging a first bone of a joint, a lower paddle for engaging a secondbone of the joint, and a displacement mechanism having a drive assemblyoperable to move the upper paddles relative to the lower paddle. Thecomputer aided orthopedic surgery system further includes referencemarkers for tracking a position of the first bone and second bone and acomputer. The computer includes a display, a processor, and a memoryhaving stored thereon software executable by the processor to track theposition of the reference markers and determine a gap spacing betweenthe upper paddles and the lower paddle throughout a range of motion ofthe joint, a varus/valgus angle between the upper paddles and lowerpaddle throughout a range of motion of the joint, and output on thedisplay an overlay of the varus/valgus angle and gap spacing throughouta range of motion of the joint.

In accordance with another preferred embodiment, the present inventionprovides a computer aided orthopedic surgery system that includes, anorthopedic distraction device and a computer. The orthopedic distractiondevice includes upper paddles for engaging a first bone of a joint, alower paddle for engaging a second bone of the joint, and a displacementmechanism having a drive assembly operable to supply displacement forcesto the upper and lower paddles. The computer includes a display, aprocessor, and a non-transitory computer-readable medium having storedthereon at least one of a femoral knee implant model and a tibial kneeimplant model and computer program instructions executable by theprocessor to cause the computer to determine a force elongation profileof a displacement between the upper and lower paddles and thedisplacement forces, and output on the display a position of the femoralknee implant model on a computer model of the first bone and thedisplacement forces on the positioned femoral knee implant model basedon the force elongation profile. The computer is also configured toadjust the position of the implant and display predicted force based onthe force elongation profile and the adjusted position of the implantand corresponding gap.

The computer further includes computer program instructions to cause thecomputer to output on the display a position of the tibial knee implantmodel on a computer model of the second bone and a displacement force onthe positioned tibial knee implant model based on the force elongationprofile. Further, the computer includes computer program instructions tocause the computer to output on the display a position of the femoralknee implant model on a computer model of the first bone, a position ofthe tibial knee implant model on a computer model of the second bone,and at least one of a position of contact force and a magnitude ofcontact force between the femoral knee implant model and the tibial kneeimplant model based on the force elongation profile. Furthermore, thecomputer includes computer program instructions to cause the computer tooutput on the display a predicted force indicative of at least one ofligament tension forces and soft tissue forces of the joint as afunction of a planned position of the implant model based on the forceelongation profile. The implant model can be a femoral implant modeland/or a tibial implant model.

The computer includes computer program instructions to cause thecomputer to output on the display a predicted force on the at least oneof a femoral knee implant model and a tibial knee implant model as afunction of gap spacing between the first and second bones based on theforce elongation profile. Further, the computer includes computerprogram instructions to cause the computer to output on the display apredicted force on the at least one of a femoral knee implant model anda tibial knee implant model as a function of flexion angle between thefirst and second bones based on the force elongation profile.

In accordance with a preferred embodiment, the present inventionprovides a total knee arthroplasty trialing system that includes aplurality of femoral trial components each having a unique surfacegeometry profile and an adjustable insert trial system. The adjustableinsert trial system includes an upper paddle, a lower paddle, adisplacement mechanism having a drive assembly operable to adjust aspacing between the upper paddle and the lower paddle between a firstposition and a second position, and a plurality of insert trial augmentsconnectable to the upper paddle, each having an upper surfacecomplementary in shape to a respective surface geometry of the pluralityof femoral trial components. Each of the plurality of insert trialaugments can have the same minimum thickness. The lower paddle includesfasteners for fastening to a bone and guide members for receiving a keelpunch. The fastener is at least one of a pin, a plug fastener, and ascrew. The lower paddle can also include at least one of a keel openingfor receiving a keel punch and a fastener opening for receiving afastener. The lower paddle is releasably connected to the displacementmechanism. The adjustable insert trial system further comprises acontroller configured to automatically adjust the spacing between theupper and lower paddles to achieve substantially equal forces on theupper and lower paddles at about full extension and at about 90 degreesflexion. Further, the adjustable insert trial system comprises acontroller configured to automatically adjust the spacing between theupper and lower paddles to achieve substantially equal forces on theupper and lower paddles throughout a full range of motion. Furthermore,the adjustable insert trial system comprises a controller configured tovary a force applied by the upper and lower paddles based on flexionangle. The adjustable insert trial system further comprises a controllerconfigured to determine a trial insert thickness based on a forceapplied by the upper and lower paddles, and a deflection factor. Acontroller of the adjustable insert trial system is configured todetermine an optimal spacing between the upper and lower paddlesthroughout a range of motion. The controller is also configured todetermine an optimal trial insert thickness based on spacing between theupper and lower paddles throughout a range of motion. The adjustableinsert trial system further comprises a reference marker for tracking aposition of the lower paddle with a three dimensional position trackingsystem.

In accordance with another preferred embodiment, the present inventionprovides a method for planning and assessing bone resections in anarthroplasty procedure of a knee joint comprising, using a computeraided orthopedic surgery system, tracking a position of a femur andtibia of the knee joint with a three dimensional position trackingsystem, resecting a proximal portion of the tibia and measuring alocation of the tibial resection, inserting a joint distraction devicehaving a lower paddle and at least one upper paddle into the knee joint,and positioning the lower paddle on the resected surface of the tibia.Using a computer aided orthopedic surgery system, controlling a forceapplied between the tibia and femur with the joint distraction device,measuring relative positions between the tibia and the femur during arange of motion of the knee joint while the joint distraction device iscontrolling the force applied between the tibia and the femur,determining an initial position and size of a 3D computer femoralimplant model on a computer model of the femur bone, calculating apredicted gap versus flexion curve between the 3D computer femoralimplant model surface and at least one of the location of the tibialresection or a surface of a planned 3D computer tibial implant model,based on the planned position of the 3D computer femoral implant modeland the measured relative positions of the tibia and femur during therange of knee flexion angles, displaying on a computer display theplanned the position of the femoral component and the calculated gapcurves, adjusting the planned femoral position or size and dynamicallyupdating the predicted gap versus flexion curve as a function of theadjusted position and/or size, resecting the femur according to theadjusted plan and inserting a femoral trial implant or actual implant,controlling the height of the joint distraction device to match theheight of the planned 3D computer tibial implant model, and using acomputer aided orthopedic surgery system, measuring forces acting on thejoint distraction device during a second range of motion of the kneejoint and displaying forces versus flexion on the display.

The method further comprises, using a computer aided orthopedic surgerysystem, varying a displacement of the upper and lower paddles of thejoint distraction device to correspond to a tibial insert thicknesssize, measuring forces acting on the joint distraction device during athird range of motion of the knee joint, displaying said forces versusflexion on the display, color coding the force versus flexion curvedisplayed on the display, wherein the color codes correspond to amagnitude of force, and registering points on the femur bone with athree dimensional position tracking system to obtain a computer model ofthe femur. The method includes adding augments to the joint distractiondevice to replicate an upper surface of the tibial implant to beimplanted in the knee joint.

In accordance with a preferred embodiment, the present invention amethod for selecting a thickness of a tibial insert implant in anarthroplasty procedure of a knee joint comprising resecting femur andtibial bones to receive femoral and tibial implants, inserting a femoralimplant on the resected femur bone, and inserting a joint distractiondevice between the resected femur and tibia bones. The joint distractiondevice includes an upper articulating surface that matches a tibialinsert upper surface, a lower plate, an automatic active spacing devicefor controlling a space between the upper surface and lower plate, andforce sensors for sensing a force between the upper articulating surfaceand lower plate. The method further includes, using a computer aidedorthopedic surgery system, controlling a spacing between of the upperarticulating surface and the lower plate of the joint distractionsystem, measuring forces between the upper articulating surfaces andlower plate during a range of motion of the knee joint, displaying themeasured forces on a display, adjusting the spacing between of the upperarticulating surface and the lower plate of the joint distraction systemand measuring forces between the upper articulating surfaces and lowerplate during a range of knee flexion angles, and selecting a thicknessof the tibial insert to implant based on the force measurements.

The method further comprises, using a computer aided orthopedic surgerysystem, controlling the spacing between of the upper articulatingsurface and the lower plate of the active ligament balancer to match athickness of the tibial implant, displaying the measured forces as afunction of flexion angle, displaying the measured forces on a computerscreen display, color coding the measured forces according to amagnitude of force, adjusting the rotation of the active ligamentbalancer on the tibial resection, tracking and storing a position of theactive ligament balancer during a range of motion of the knee joint. Themethod also includes guiding a location of a tibial keel punch with thelower plate of the active ligament balancer.

In accordance with yet another preferred embodiment, the presentinvention provides a method for planning and assessing bone resectionsin an arthroplasty procedure comprising receiving bone morphology dataof a joint, joint gap data and relative position data of a first boneand a second bone of the joint, receiving user input data indicative ofan applied distraction force, controlling the applied distraction forcesupplied by a joint distraction device according to the received userinput data, adjusting the applied distraction force as a function ofrelative position of first and second bones, recording relativepositions of the first and second bones of the joint while controllingthe applied distraction force, positioning and sizing at least one of afirst implant on the first bone and a second implant on the second bone,based off of the recorded relative positions of the first and secondbones, determining a position and size of an implant on at least one ofthe first and second bones, determining a position and size of a firstimplant on the first bone and a second implant on the second bone,displaying the determined position and/or size of the implant on the atleast one of the first and second bones. Displaying the determinedposition and/or size of the first implant on the first bone and thesecond implant on the second bone, determining a resection depth of thefirst bone based on the determined position and/or size of the firstimplant and a resection depth of the second bone based on the determinedposition and/or size of the second implant, displaying a predictive gapbetween the first implant on the first bone and the second implant onthe second bone, displaying a predictive gap between the first andsecond bones, displaying a predictive gap between the implant on one ofthe first and second bones, and the other of the first and second bones,receiving a user input to adjust the position and/or size of the implanton one of the first and second bones, displaying resection depths, andgaps between the first and second bones, based on the received userinput to adjust the position and/or size of the first or second implant,positioning a robotic arm or cutting guide to the determined resectiondepth of the first or second bone, receiving user input on a selectedthickness of a tibial implant as the second implant for the second bone,controlling a height of the joint distraction device based on theselected thickness of the tibial implant, sensing a force acting on thejoint distraction device at the controlled height position whilemeasuring the relative position of the first bone and second bone,displaying the first implant on the first bone and/or the second implanton the second bone, displaying the force acting on the distractiondevice on a display and displaying the relative positions of the firstand second bones on the display, and/or displaying a graph of forceversus flexion angle.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there are shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown.

In the drawings:

FIG. 1 is a schematic diagram of a computer aided orthopedic surgerysystem in accordance with a preferred embodiment of the presentinvention;

FIGS. 2A-D are various views of an orthopedic distraction device inaccordance with a preferred embodiment of the present invention;

FIG. 2E is a cross-sectional anterior elevation view of the orthopedicdistraction device of FIG. 2A;

FIGS. 2F and 2G are views of a flexure bracket of the orthopedicdistraction device of FIG. 2A;

FIG. 2H is perspective view of the orthopedic distraction device of FIG.2A with parts omitted for purposes of illustration;

FIG. 2I are views of various components of orthopedic distraction deviceof FIG. 2A;

FIGS. 2J-M are various view of internal components of the distractiondevice of FIG. 2A;

FIGS. 3A-3D are views of an orthopedic distraction device in accordancewith another preferred embodiment of the present invention;

FIGS. 4A and 4B are perspective views showing the internal components ofthe orthopedic distraction device of FIGS. 2A and 3A, respectively;

FIG. 5A is a representation of a force vs. elongation relationshipmeasurement acquired by the orthopedic distraction device of FIG. 2A;

FIG. 5B is a representation of the height vs. time constant velocitycontrol mode of orthopedic distraction device of FIG. 2A;

FIG. 5C is a representation of the force vs. time relationship for theconstant force-velocity control mode of orthopedic distraction device ofFIG. 2A;

FIGS. 6A and 6B are side and oblique views of the orthopedic distractiondevice of FIG. 3A inserted in a knee joint in a flexed position;

FIG. 7 is an illustration of force vs. flexion profile applied by theorthopedic distraction device of FIG. 2A;

FIG. 8 is a representation of a femoral and tibial planning userinterface in accordance with an aspect of the computer aided orthopedicsurgery system;

FIG. 9A is a screen shot view of a pre-operative kinematics acquisitionuser interface in accordance with an aspect of the computer aidedorthopedic surgery system;

FIGS. 9B and 9C are screen shot views of a ligament balancing userinterface in accordance with an aspect of the computer aided orthopedicsurgery system;

FIG. 9D is an illustration of an applied force vs. flexion profilescreen of the ligament balancing user interface of FIG. 9B;

FIG. 10A is screen shot view of a post-resection stability assessmentuser interface for height mode in accordance with an aspect of thecomputer aided orthopedic surgery system;

FIG. 10B is screen shot view of a post-resection stability assessmentuser interface for force mode in accordance with an aspect of thecomputer aided orthopedic surgery system;

FIG. 10C is screen shot view of a femoral and tibial planning userinterface in accordance with an aspect of the computer aided orthopedicsurgery system;

FIGS. 11A-F are in sequence views showing the operation of the lowerpaddle of the distraction device of FIG. 2A being fixed to a tibia;

FIGS. 12 A-D are views showing the operation of the lower paddle beingfixed to the tibia while allowing for rotation and/or translation of thelower paddle and ligament balancer relative to the tibia;

FIG. 13A shows a process flow chart overview of the computer aidedorthopedic surgery system in accordance with an aspect, used in a tibialcut first ligament balancing technique;

FIG. 13B shows a process flow chart overview of the computer aidedorthopedic surgery system in accordance with an aspect; used in a femurcut first technique;

FIG. 14A is a partial view of an orthopedic distraction device inaccordance with another preferred embodiment of the present inventioninserted within a knee joint;

FIG. 14B is a partial view of the orthopedic distraction device of FIG.14A in accordance with another aspect of the embodiment;

FIG. 14C is a partial view of the orthopedic distraction device of FIG.14A in accordance with yet another aspect of the embodiment havingpaddles with a plurality of struts;

FIG. 15 is a side view of an orthopedic distraction device in accordancewith yet another preferred embodiment of the present invention insertedwithin a knee joint;

FIG. 16 illustrates a kit in accordance with another preferredembodiment of the present invention;

FIG. 17A is a screen shot view of a ligament balancing user interface inaccordance with an aspect of the computer aided orthopedic surgerysystem of the present invention;

FIG. 17B is a screen shot view of an implant planning user interface inaccordance with another aspect of the computer aided orthopedic surgerysystem;

FIG. 17C is a screen shot view of a post-operative kinematics userinterface in accordance with an aspect of the computer aided orthopedicsurgery system;

FIG. 18 is a perspective view of a robotic system of the computer aidedorthopedic surgical system of the present invention having a robotic armattached to a distraction device; and

FIG. 19 is a perspective view of a robotic system of the computer aidedorthopedic surgical system of the present invention having a robotic armwith an integrated distraction device on its distal end.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention illustrated in the accompanying drawings. Wherever possible,the same or like reference numbers will be used throughout the drawingsto refer to the same or like features. It should be noted that thedrawings are in simplified form and are not drawn to precise scale. Inreference to the disclosure herein, for purposes of convenience andclarity only, directional terms such as top, bottom, above, below anddiagonal, are used with respect to the accompanying drawings. The term“proximal” refers to being nearer to the center of a body or a point ofattachment. The term “distal” refers to being away from the center of abody or from a point of attachment. Such directional terms used inconjunction with the following description of the drawings should not beconstrued to limit the scope of the invention in any manner notexplicitly set forth. Additionally, the term “a,” as used in thespecification, means “at least one.” The terminology includes the wordsabove specifically mentioned, derivatives thereof, and words of similarimport.

“About” as used herein when referring to a measurable value such as anamount, a temporal duration, and the like, is meant to encompassvariations of ±20%, ±10%, ±5%, ±1%, and ±0.1% from the specified value,as such variations are appropriate.

Ranges throughout this disclosure and various aspects of the inventioncan be presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, and 6. Thisapplies regardless of the breadth of the range.

Furthermore, the described features, advantages and characteristics ofthe embodiments of the invention may be combined in any suitable mannerin one or more embodiments. One skilled in the relevant art willrecognize, in light of the description herein, that the invention can bepracticed without one or more of the specific features or advantages ofa particular embodiment. In other instances, additional features andadvantages may be recognized in certain embodiments that may not bepresent in all embodiments of the invention.

In accordance with preferred embodiments and aspects of the presentinvention, there is provided the following:

Computer Aided Orthopedic Surgery System

In accordance with a preferred embodiment, the present inventionprovides computer-assisted orthopedic surgery (CAOS) 1000, or navigationsystem, FIG. 1, for joint replacement or resurfacing procedures, such asknee arthroplasty procedures including unicondylar knee arthroplasty(UKA), total knee arthroplasty (TKA), or revision TKA. Although thesystem is primarily described in the context of knee arthroplasty, itshould be understood that the system could be used for other surgicalprocedures, such as hip, ankle, shoulder or elbow arthroplasty, orligament reconstruction procedures such as Anterior or PosteriorCruciate Ligament (ACL or PCL) reconstructions, and Medial or LateralCollateral Ligament (MCL or LCL) reconstructions.

The CAOS system preferably includes a three dimensional (3D) positiontracking system 2, for tracking the positions of the patient's bones andsurgical instruments in 3D space. Any technology for position trackingmay be used, including optical, electromagnetic, ultrasonic,radiofrequency, or accelerometer based tracking systems. Opticaltracking systems usually use passive (retro-reflective) or active (LED)markers which are attached to the bones (for example, tibial referencemarker 107 and femoral reference marker 106 shown in FIG. 6A) andtracked using an optical camera that is in communication with a centralcomputer 4 of the CAOS station. Non-invasive tracking systems may alsobe used, such as transcutaneous ultrasound based tracking technology, orby tracking markers on the skin of the patient and compensating formotion of the skin relative to the underlying bones.

The CAOS system includes capabilities for establishing a coordinatesystem, such as a Cartesian coordinate system (x,y,z), associated witheach bone. The coordinate system can correspond to the directions of theanatomical planes of each bone (x=anteroposterior, y=mediolateral,z=proximodistal). The CAOS system further allows for registering theanatomy of the patient's bones and in particular the anatomy in thevicinity of the patient's joint to be operated on, as well as themechanical axis of the patient's leg, including the tibial mechanicalaxis 99, femoral mechanical axis 98, and overall mechanical axis of theleg (see FIG. 6A). The CAOS system also includes capabilities forgenerating a computer model of the patient's joint, using eitherinformation from pre-operative images, or by using generic models thatare not specific to the patient but created or deformed to match thepatient anatomy acquired or digitized in the OR. CAOS systems andmethods for creating computer bone models applicable to the presentinvention are disclosed e.g., in U.S. Pat. Nos. 8,126,533 and 9,248,001,the entire disclosures of which are incorporated by reference herein forall purposes.

The CAOS system may include a probe for scanning the surface of thebones, such as a point probe that is physically touched to or slid alongthe bone surface while its position is being tracked relative to thebone by the 3D tracking system, or an echographic probe for collectingpoints through the skin and underlying soft tissues. The CAOS system canalso include instruments for measuring the location of bone cut surfacesmade in the bone for receiving an implant. For example a plate or planarprobe (as known as a cut controller) can be used to measure the 3Dlocation, angles and depths of bone resections such as the tibialresections and proximal femoral resections.

The CAOS system includes a central computer 4 for computing data and forconnecting peripherals, including the tracking system, and a display 6or multiple displays for displaying information in the OR. Any type ofdisplays may be used, including 2D or 3D computer monitor screens, orheads-up and/or head-mounted displays. The display may also be a touchscreen allowing the user to enter data and provide various controlinputs to the system. Additionally, a remote control 7 may be used, suchas a battery operated handheld wireless remote control device withbuttons that can either be held in hand, placed on the OR table, orattached to a surgical instrument or an orthopedic distraction device 1(also referred to herein as a ligament balancer). The remote control 7may also be a wireless tablet computer with a touchscreen that can beeither held by a non-sterile user (for example a nurse or technicalsupport staff), attached to a CAOS workstation, or draped with steriledrapes and placed directly in the surgical field (for instance,attaching to the surgical table), allowing the surgeon or surgeonassistant to control the system. The remote control may also be a footswitch or foot pedal that is either in wireless or wired communicationwith the computer 4.

The computer and/or controller includes a processor and a memory havingstored thereon software or computer instructions for planning the jointreplacement procedure, including algorithms for planning the position ofimplants on the patients bones based off of bone morphology data and offof ligament data. The software and algorithms of the CAOS in accordancewith the present embodiments are further discussed below. The softwaremay include modules for assessment of the final ligament balance of thesurgical procedure once the implants are in place. As used herein todescribe the configuration of the controller or computer, configured tomeans that the controller or computer includes software or computerinstructions stored in memory executable by the processor to cause thecomputer to function and operate as specificed.

The CAOS system may include a robot 8 for executing the bone resectionsaccording to the plan. The robot may be floor mounted, table mounted,bone mounted, or handheld and may be programmed to provide autonomous orhaptic guidance of the resections using various tools such asreciprocating, oscillating or rotating cutting tools, including bonesaws, blades, burrs, mills, or reamers, or energy based (laser) cuttingtools. Exemplary robots applicable to the present invention includethose disclosed in U.S. Patent Application Publication No. 2011/0130761and U.S. Pat. No. 8,840,629, the entire disclosures of which areincorporated by reference herein in their entirety for all purposes.

In accordance with another aspect, the CAOS system 1000 includes arobotic system 1010 having a robotic arm 1012. The robotic system, forexample, can be programmed with a three-dimensional virtual region ofconstraint that is registered to a patient and the robotic arm can beconfigured to include three or more degrees of freedom. Robotic systemsapplicable to the present invention includes those disclosed in U.S.Pat. Nos. 9,002,426 and 7,747,311, the entire disclosures of which arehereby incorporated by reference herein in their entirety for allpurposes. The robotic system 1010 is operatively in communication withthe computer 4, programmable to carry out predetermined task and/orfunctions.

As shown in FIG. 18, the robotic system 1010 includes the orthopedicdistraction device or ligament balancer 1, as further described below.That is, the distraction device is attached to the end of the roboticarm for control and manipulation of the distraction device. In thismanner, cables extending from the distraction device can be run throughor integrated into the robotic arm, or attached to the robotic arm,either internally or externally of the arm's outer housing.

In operation, the robotic arm is used to support and position thedistraction device in the knee joint, thus providing the ability tocompensate for the weight of the distraction device during use. Further,to maintain the sterile field in the OR, the robotic arm can be drapedso as to forego the need to sterilize the robotic arm. When draped withsterile draping 1014, the distraction device can be sterilized andattached to the robotic arm on top of the sterile drape using dedicatedcouplings. Alternatively, the distraction device can be non-sterile andattached to the robotic arm under the sterile draping, with the upperand lower paddles of the distraction device are sterilized and extendingout through the sterile draping.

In accordance with another aspect, the distractor 1016 and force sensorsare integrated into the distal end of the robotic arm, i.e. built intothe arm, as shown in FIG. 19. In other words, the distal end of therobotic arm 1016 is equipped with one or more force sensors 1020 and oneor more actuators 1022 that are configured to distract the bones of thejoint, and function as the distractor in any of the embodiments andmodes of control and function below mentioned (force control, heightcontrol, force-height control, disabled and enabled, virtual trailing,and so on). The upper and lower paddles 1018 can be modular and attachedto the distractor (or end-effector) of the robotic arm, so thatdifferent designs of paddles may be attached for different purposes. Therobotic arm 1010 with the distractor 1016 can be draped with a drape1014 to keep the field sterile, and the paddles may be attached to therobotic arm end effector (distractor) through the drape. Force sensors1020 can be integrated into the distractor mechanism behind the drape1014 so that they do not need to be sterilized. The distractor can beconfigured to distract using linear sliding joints as shown in FIG. 18,or rotational joints that generate relative parallel motion of thepaddles via a parallel linkage mechanism 1024 (FIG. 19), to keep theupper and lower paddles 1018 parallel to one another during distractionor post-resection trialing. Any lateral movement of the paddles relativeto the bones that is created as a result of the actuator 1022 andparallel linkage mechanism 1024 moving the paddles closer or furtheraway from one another can be compensated for by motion of the otherjoints 1026 of the robotic-arm 1012. The robotic arm can be mobile andhave wheels that allow it to be moved on the floor, or it can be mounteddirectly to the operating room table. For example, the base of the robotcan be clamped on the side rails of the table and can be light andportable so it can be easily transported from OR to OR or from hospitalto hospital. The end effector can be configured to be a distractor aswell as a robotic gripper, which would allow it to grip or hold othertools such as a bone cutting burr or saw for cutting the bones of thejoint for inserting the implants according the plan generated by theCAOS system and the data acquired with the distractor.

The orthopedic distraction device or ligament balancer 1 is incommunication with the central computer 4, and controllers 3 forcontrolling the motion and function of the ligament balancer. Theligament balancer includes a drive assembly, e.g., actuators 131 (FIG.4A), for actuating the balancer, and sensors for sensing the forcesacting on the ligament balancer. The actuators are preferably electricmotors, however, any known actuator could be used, includingpiezo-electric, pneumatic, hydraulic, magnetic/induction, spindle driveand the like. The drive assembly is operable to move the upper paddlerelative to the lower paddle. That is, the drive assembly displaces thespacing between the upper and lower paddles under the control of thecomputer i.e., a controller. In other words, the drive assembly isoperable to move one of the upper and lower paddles relative to theother of the upper and lower paddles. The drive assembly is preferably ahermetically sealed drive assembly, as further described below.Alternatively, the drive assembly can be other types of drive assembliessuitable for the intended purpose of the present embodiments, e.g., ahydraulic drive assembly or a balloon drive assembly, as applicable toall or particular embodiments disclosed herein.

As referred to herein, the orthopedic distraction device can include acontroller separate from a computer, or a computer functioning as acontroller. That is, the functions and capabilities of the presentinvention described herein can be embodied in a computer separate from acontroller or a single controller embodied as a controller.

Referring now to FIGS. 2A-E, various views of the ligament balancer 1are shown. In FIG. 2A, a perspective view of a preferred embodiment ofthe knee ligament balancer 1 is shown. The ligament balancer 1 includesa displacement mechanism 5, an upper paddle 20 and a lower paddle 12. Inaccordance with an aspect, the upper paddle 20 can includes a firstupper paddle 21 and a second upper paddle 23. The first upper paddle canbe a medial upper paddle and the second upper paddle can be a lateralupper paddle. The upper paddle is configured to engage a first bone of ajoint and the lower paddle is configured to engage a second bone of thejoint.

FIGS. 2J-M illustrate various components of the distraction device. FIG.2J shows a top part 5 a of the housing 5. FIGS. 2K-M show the internalstructure of the distraction device include the top part 5 a, interiorchassis 5 b connected to the top part, and center supports 5 c and 5 d,which are all rigidly connected to each other.

The upper medial and lateral paddles have surfaces 22, 24 that areintended and adapted to contact and articulate with the medial 101 andlateral 102 condyles of a femur 100 (FIGS. 2D and 6A-B). Contactsurfaces 22, 24 may be flat or curved/concaved, and may be smooth toallow for sliding of the bone on the paddle contact surface. Whenaugments are attached to the upper paddles, the contact surfaces arebelow the augments, and preferably directly below the augments.

Referring to FIG. 2I, the ligament balancer can include a plurality oflower paddles of varying sizes and for either the right or left knee.Each of the plurality of lower paddles are sized and shaped to match arespective size and shape of a plurality of implants (e.g., a pluralityof tibial implants, as shown in FIG. 16) to be implanted in the tibiae.g., a second bone.

The lower paddle 12 has a surface that is intended to contact the tibia105. The lower paddle preferably has a lower surface or undersurface 13(FIG. 2D) that is substantially flat and intended to sit on top of atibial cut surface 110 (FIG. 6B). The surface 13 can have surfacetexture or geometric features which help it engage and grip the tibialcut surface, such show generally so that the ligament balancer does notslip on the cut surface during use of the device. In some cases, it isdesired that the ligament balancer stay in place in the knee and inparticular on the tibial cut during use and during various kneestability tests and motions, which can include varus/valgus stability orstress tests, continuous gap or force acquisitions throughout a range ofmotion of the joint in a range of flexion angles, heel-push tests, etc.,without requiring the surgeon to hold the device in place by hand. Toincrease the stability of the ligament balances within the knee, theligament balances could be fixed to the tibia using the lower paddle 12.

As can be seen in FIG. 2D, the ligament balancer is reconfigurable andmodular, allowing the attachment and de-attachment of different sets ofupper and lower paddles from the displacement mechanism, to permit theuse of different sizes and shapes of upper and lower paddles toaccommodate the range of patient joint anatomies and sides (right orleft). The upper medial and lateral paddles may also have features thatallow medial and lateral augments 42, 44 to be attached or easilyclipped on them, to augment the height of the paddle, or to provide adifferently shaped articulating surface for articulating with the femuror femoral component (implant or trial component). These augments canattach to the paddles using various mating features such as locatingpins 45 and holes 46 (FIG. 2A), magnets, quick release clips, and thelike. In other words, each of the plurality of augments are releasablyconnectable to the upper paddle.

The augments are preferably configured with a concave upper surface.These augments can have different levels of curvature or congruency forengaging or mating with a first bone of a joint, e.g., the native femur,or a femoral trial or actual implant once in place. That is, each of theplurality of augments is configured to articulate with the first bone(e.g., femur) or a femoral trial implant. In particular, an array ofdifferent sized augments can be provided to match the radii of curvatureof the various sizes of tibial and femoral implants provided with theimplant system. Preferably a range of different sizes of augments areprovided so that each size in the range matches the size and shape ofeach tibial insert trial implant or each tibial insert implant in therange offered in the implant system that is to be implanted in thepatient. The spacing between the medial augment and the lateral augmentis such that when they are mounted on their respective medial andlateral upper paddles, they match the spacing of the medial and lateralplateaus or dishes of the tibial insert implant to be implanted.

As shown in FIG. 2D, the ligament balancer includes an attachmentinterface 16 for attaching the lower paddle 12 to the displacementmechanism. The attachment interface can include any type of couplingmeans, such as fasteners, one or multiple screws 19, locating pins andholes, magnets, quick release clip mechanisms, and the like. As such,the lower paddle 12 is releasably connected (i.e., connectable) to thedisplacement mechanism 9.

Preferably, the ligament balancer comes with a range of different sizelower paddles 12, wherein each size in the range of lower paddle sizesmatches the corresponding size (profile, shape, medial-lateral size,anterior-posterior size, and/or thickness) of the tibial baseplate ofthe implant system being implanted. As shown in FIG. 11A, the lowerpaddle 12 can also be used as a template for the surgeon to place on thetibial bone cut in order to determine the optimal size, position and/orrotation of the tibial implant baseplate to use for that patient. Thusthe lower paddle can be placed on the tibial cut surface (attached ordetached from the ligament balancer), and because the lower paddlematches the sizes and shape (or profile) of the tibial implant, thesurgeon can select the size and rotate and position the lower paddle onthe tibial cut surface so that the outer contour of the lower paddle 12best matches the contour of the bone resection 110.

The lower paddle 12 may also include features and fasteners forfastening the paddle to the tibial resection of e.g., a second bone,such as holes 40 for receiving bone pins 375 (FIG. 11B) or screws. Thelower paddle 12 may also include features such as openings or apertures,such as a fastener opening 40 and a keel opening 41, and guide memberse.g., guide-holes or pegs 364 for receiving/guiding a keel punch 365 orother cutting or drilling tool, for creating a cavity 366 for the keelor stem of the tibial implant. The fastener and keel openings areconfigured to receive a corresponding keel punch and fastener. Thefeature or pegs 364 for guiding the tool for creating the tibial keel orstem cavity 366 is preferably positioned on the lower paddle 12 suchthat when the tibial keel or stem cavity 366 is created, the position ofthe final tibial implant will match the position of the lower paddle 12when the cavity was created. As shown in FIGS. 11C-F, once the cavity366 for the implant keel is created, a temporary plug 367 may beinserted (FIGS. 11D and 11E) into the cavity 366 to fix the lower paddle12 to the tibia. This can be used to supplement the pin 375 fixation, orinstead of the pin fixation. Using the plug instead of the pins has theadvantage of minimizing the amount of holes placed in the bone, reducinginvasiveness, since the cavity for the keel needs to be createdregardless. The ligament balancer may be attached to the lower paddleeither before or after (FIG. 11F) the lower paddle is fixed to thetibia. The ligament balancer may also be attached to the tibia, or to aleg positioner, using straps.

In another embodiment, the under surface 13 of the lower paddle issmooth to allow for rotation of the ligament balancer on the tibial cutsurface during a range of knee motion from flexion to extension, or fromextension to flexion.

The ligament balancer with lower paddle attached, or the lower paddle byitself, can be tracked relative to the bone, by attaching a referencemarker 113 to it tracking it relative to the bone. The lower paddle 12can be navigated into position using the bone model and software thatallows the surgeon to plan the position of the implant and the resectionon the bone. The knee joint may also be taken through a range of motionwith the tracked ligament balancer in the knee, and the position of theligament balancer relative to the femur and tibia may be tracked duringthis range of motion.

Because the computer 4 is able to control the height or spacing of eachupper paddle relative to the lower paddle of the ligament balancer(i.e., the space between the upper paddles and the baseplate), byclipping on a specific augment size and baseplate size, and bycontrolling the height according to the desired thickness of the insert,any implant size and thickness offered in the implant system can beconstructed, simulated, and trialed in the joint, without having to makeavailable each of the individual tibial implant sizes and thicknesses inthe operating room. Since conventional instruments usually include onetrial instrument for each implant in the range of available implantsizes and thicknesses, an advantage of the present invention is thus areduction in the total number of instruments required to be provided inthe OR.

As shown in FIG. 2C, the upper medial 21 and lateral 23 paddles can bedifferent shapes with respect to one another. For example, to facilitatea medial approach to the knee joint, the medial paddle 21 may be shorterin length and the lateral paddle 23 may be longer to reach the farlateral side. That is, the first upper paddle (shown as paddle 23 inFIG. 2C) extends further from the displacement mechanism than the secondupper paddle (shown as paddle 21 in FIG. 2C).

The upper paddle arms may also have curved profiles to avoid impingementwith the soft tissue around the knee. Particularly, the profile of thelateral arm may by curved or have a concave relief i.e., an inwardlyextending relief for clearance 18 to avoid impingement with the patellartendon and lateral displacement of the patella (FIG. 6B) when theligament balancer is inserted in the knee and when the knee is broughtinto different flexion angles. That is, one of the first and secondupper paddles includes an inwardly extending relief for clearance ofsuch ligaments, tendons or other tissue. Similarly the medial arm mayhave a curved, concaved surface/relief 20 to prevent impingement withthe medial collateral ligament and other medial tissues surrounding theknee. Different paddles can be provided for a left or a right knee. Forexample, FIG. 2C illustrates upper paddles configured for use with aleft knee, while a mirror image configuration of the upper paddles canbe used for a right knee.

Referring to FIG. 2C, owing to the angle at which the augments arealigned and attached to the upper paddles, a longitudinal axis of theaugments (A) extends at a non-perpendicular and non-parallel anglerelative to a coronal plane (B) of the displacement mechanism.

Alternatively, the paddles can be designed such that they can be swappedor interchanged from the left 31 and right 30 paddle connectors (FIG.2D) so that the same paddles can be used for a left or right knee (i.e.,the shorter paddle can be used as the medial paddle, for both a leftknee and a right knee, and the longer paddle can be used as the lateralpaddle, for both a left knee and right knee). This can be accomplishedby making the paddles symmetric, or changing the side each paddle mountson the displacement mechanisms and the angle at which it mounts on withrespect to the long axis of the device (i.e., flipping each paddleupside down). Shorter and longer paddles can be provided for smaller andlarger (i.e., obese) patients. Similarly, paddles that have a wider andnarrower overall mediolateral dimension when assembled on the ligamentbalancer can be provided to fit wider or narrower femurs and tibias andto simulate smaller and larger tibial implant sizes. The paddleconnectors are attached to the bellows shaft and thus move relative tothe drive mechanism or drive assembly.

The paddle connectors 30, 31 may include features for coupling the upperpaddles in multiple positions with respect to the displacementmechanism, such as multiple holes or slots for accommodating the samelocating pin so that the upper and lower paddles can be mounted on theconnectors such that they are further apart from one another in themedial-lateral direction, or so that they extend shorter or longer intothe joint. By allowing multiple positions of attachment for each paddle,it is not necessary to provide different paddles for fitting differentsizes of knees or for simulating smaller and larger sizes of tibialtrials and implants. The user simply has to assemble the device so thatthe appropriate locating pin is in the appropriately positioned hole orslot for simulating the desired size of knee or tibial insert trial orimplant. The paddle connectors are also hermetically enclosed bybellows.

In accordance with another embodiment, the orthopedic distraction devicecan be configured with the lower paddle 12 is connected to thedisplacement mechanism using a quick disconnect system 10, as shown inFIGS. 3A-3D. Buttons 14 (FIG. 3A) on either side of the lower paddlerelease the lower plate from the body, by using a dove tail or T-slotsliding rails 15 (FIG. 3B) with an axial catch that is released when thebutton is pressed. Buttons 14 can be provided on one or both sides ofthe device to allow for the quick and ergonomic release of the lowerpaddle.

Sealing

Referring back to FIGS. 2A-E, the displacement mechanism 5 includes ahousing or body 9 that forms an enclosure that is preferably sealed, andmore preferably hermitically sealed. O-ring and other seals can beprovided as static seals at the cable connector 32 and at the upper part33 of the body which provides access into the main body. The linear axesof the paddle connecters are preferably sealed by the use of bellows 17of a bellows assembly, which allow the linear motion (expansion andcontraction) of the paddles with respect to the displacement mechanismand the lower paddle, while maintaining a complete seal of thedisplacement device. Thus, the bellows assembly provides forhermetically sealing the drive assembly to the displacement mechanism.For example, the bellows assembly is connected to the upper paddle anddrive assembly thus forming a sealed enclosure, as shown in FIG. 4A.

The bellows assembly includes bellows 17 and a bellows shaft 133. Oneend of the bellows is connected to a top end of the bellows shaft and anopposite end is connected to the housing 5, e.g., a top end of thehousing. The bottom end of the bellows shaft 133 is connected to aball-screw bearing 132, and preferably rigidly connected to an outersurface of the ball-screw bearing. The bellows assembly is moveablerelative to the drive assembly 131

By expansion and contraction of the bellows assembly along the axis ofthe bellows, the paddles can move up and down (or further or closer tothe lower paddle) along the same axes while the ligament balancermaintains a sealed state. This allows the body 9 to be washed andcleaned and sterilized in an autoclave, and will prevent steam vapor orcleaning agents from entering inside the device and affecting thefunction and performance of the internal mechanisms. It also preventsany contaminates from coming out of the displacement mechanism andinfecting the patient or compromising the sterile field.

The bellows 17 can be metal and manufactured with precision welding orlaser welding operations. Metal bellows provide greater durability.Alternatively, the bellows can be made out of plastic and could beinjection molded to reduce costs.

The bellows assembly includes paddle connectors 30, 31 which may includeflanges or other features to make it easier for a user to grip theconnectors and pull them up and expand the bellows 17 when the upperpaddles are attached. This would facilitate access to the outer surfacesof the bellows for cleaning of the ligament balancer after use insurgery.

Motion

The CAOS system 1000 includes components that allow for active motionand control of the displacement device 1. Referring now to FIG. 4A, aview of the ligament balancer is shown with a transparent body 9. Thecomponents that allow for active motion include a drive assembly 131having motors 126, gear heads such as planetary gear heads 127, and ballscrews. The ball screw and linear guide, which includes rails 155 andcarriages 150, translate the rotatory motion of the motors 126 and gears127 into linear motion of the carriages 150, which are connected to thepaddle connectors 31, 30, which are in turn connected to the upperpaddles 21, 23. The motors can have hall sensors (otherwise known ashall-effect sensors) that control the communication of the electricalsignals and power to the motor windings. The system may also include anencoder 105 to monitor the rotational position of the motor.

The drive assembly 131 also includes a plunger 131 a operatively drivenby the motor 126. The plunger can be a spindle or threaded plunger.Operation of the motor 126 rotates the plunger, which in turn engages aball-screw bearing 132. The ball-screw bearing is connected to rails 155and carriage 150 that supports the bellows assemblies and paddleconnectors. Thus, the bellows assembly translates in an axial directiondue to the rotation of the plunger 131 a.

The drive assembly is controlled by the controller 4, which isconfigured to apply a first displacement force to a first upper paddle(e.g., a medial upper paddle) and a second displacement force to asecond upper paddle (e.g., a lateral upper paddle). The seconddisplacement force can differ from the first displacement force. Inother words, the displacement mechanism is configured to independentlydisplace or apply a displacement force to each of the first and secondupper paddles.

Alternatively, the hall sensors can be used to monitor the rotationalposition of the motors. Either encoders or hall sensors can be used formonitoring the position of the motors and thus can be used forcontrolling the position of the motors and, through the transmissionsystem (i.e., the gears, ball screws, and sliders), the paddles.

Alternatively, the analogue power channels (lines) from the controllerto the motor that are used to power each individual motor winding can beused to provide information on the rotational position of the motors. Inparticular, the relative phase and the sinusoidal shape of each channel(line) that powers the motor windings can be used to estimate therotational position of the motors. This has the advantage of asensorless motor control approach which requires fewer wires and thussimpler cables and hardware components. The motors can be powered with apower supply that is connected to the main network power, where thepower supply is connected to the ligament balancer via the controllers3, a cable 134 and cable connector 32, or the motors may be batterypowered so that a cable is not required, allowing for wirelesscapabilities. The motor controllers can be integrated into thedisplacement mechanism of the ligament balancer to allow for wirelesscontrol, where command signals are sent from an emitter connected to thecomputer, wirelessly through the air via the electromagnetic spectrum,to a receiver and the integrated controllers into the ligament balancer.

Any type of wireless communication protocol and communication hardwaremay be used. To facilitate accurate and controlled motion of theligament balancer 1, the ligament balancer may be homed either before orafter it is assembled, or at any time during the procedure. Homing shallmean correlating the rotational position of the motors to the linearposition of the upper and lower paddles, such that the positionalrelationship between these two is known. Homing the ligament balancermay be accomplished by automatically moving the upper paddles down untilthey come into contact with the lower paddle while measuring theposition of the motors, thus determining a reference or zero position.During the homing sequence, current to the motors may be monitored orlimited, or the force sensors (described below) may be used, to reduceor limit the force the device may apply during the homing motion, thuspreventing any pinch hazards for an operator's fingers or the patient'ssoft-tissues.

Force Sensors

The ligament balancer 1 also includes force or load sensors 130 tomeasure the loads acting on each of the paddles. These sensors can beforce sensing resistors, or any other sensor technology known in theart, such as piezo-electric, piezoresistive, strain-gage based, thin orthink film sensors, capacitive wave-guide technology, and the like.

The force sensors are preferably mounted in the sealed body 9 of thedisplacement mechanism 5 to shield them from the environment duringcleaning, sterilization and use during in surgery. They can be mountedunder the motors or drive assembly at the base of the body such thatwhen a load is applied at the paddles, the force is transferred throughthe bodies of the bellows assembly and drive assembly e.g., linearguides, ball screws, gears, motors and encoders. In other words, thesensors are positioned within the housing and below the drive assembly.

The displacement mechanism further includes a flexure motors bracket orflexure bracket 125 (FIGS. 2E, 2F, 2G and 2H) used to rotationally fixor constrain the two motors relative to each other, and/or relative tothe housing body 9 and other internal components of the displacementmechanism, while still allowing axial force to be transmitted throughthe flexure bracket to the force sensors 130 underneath the motors atthe base of the body. The flexure bracket 125 is designed to allow forsome flexion of the bracket, to allow for some small axial motion of themotors relative to the body, while rotationally constraining the motorand gear housings to allow the motors to transmit torque to ball-screwbearings 132 and output shafts or bellows shaft 133. Thus the forcesensors are axially positioned under the motors and are subjected to theforces acting on the paddles by virtue of the flexing of the flexurebracket 125.

The flexure 125 is configured as best shown in FIGS. 2F and 2G, andincludes a rigid portion 128 and a flexure portion 129. The flexureshown in the present embodiment is configured to support two separatedrive assemblies, but can alternatively be configured to support asingle drive assembly, or more than two drive assemblies, or formed astwo separate flexure brackets, each one configured to support anindividual drive assembly. The rigid portion 128 is fixedly mounted tothe housing 9 of the displacement mechanism 5. For example, the rigidportion can be fixed to the housing by fasteners extending throughapertures 123, which can be aligned to corresponding apertures on thehousing.

The drive assembly is mounted to the flexure portion 129 via fasteners,as best shown in FIG. 2H. The flexure portion includes apertures 124 forreceiving said fasteners. Preferably, a top most portion of the driveassembly is mounted to the underside of the flexure portion. Thus, owingto the flexure design of the flexure bracket, as best shown in FIG. 2Fthe flexure bracket allows for movement (e.g., minor movement due todeflection of the flexure portion) of the attached drive assemblyrelative to the rigid portion in at least one direction e.g., an axialdirection. In other words, the drive assembly is axially movable betweena first position and a second position spaced from the first position.

In sum, the displacement mechanism includes a housing body and flexurebracket connected to the housing body. The flexure secures the driveassembly within the housing body, thus allowing the drive assembly tomove between a first position and a second position spaced from thefirst position, e.g., axially spaced. Further, owing to the positioningof the sensor within the housing, the drive assembly engages the sensorin both the first and second positions. That is, when the drive assemblyaxially moves between the first and second positions, it maintains itsengagement with the sensor at all time.

The force sensors 130 are configured to generate an electrical signalthat is transmitted to the computer 4 and controllers 3 that isindicative of the force acting on the sensor and paddles. The electricalsignal may be amplified by an amplifier and converted from an analoguesignal to a digital signal via an analogue to digital (A2D) converter.The force signals can be transmitted wirelessly or by wire. In order toimprove the force sensing resolution of the system, two force sensorscan be used on either side of the balancer, where one sensor isoptimized to read forces in one range (for example 0-150N) and thesecond sensor is designed to read forces in a second range (e.g.,100-500N). This way, the appropriate sensor may be read depending onwhat range the force is, providing a more accurate measurement and alsoproviding some redundancy in the system to better detect faults andmalfunctions. The sensors may be stacked, with intermediate members inbetween them, such as a smooth shim, to provide optimal sensing surfaceon both sides of each sensor.

The controller 4, which is operatively in communication with thedisplacement mechanism, is configured to move the displacement mechanismto receive a predetermined load force. The predetermined load force canbe entered and stored in the controller and set to a particular userpreference. The controller can also be configured to measure the loadforce applied to at least one of the medial upper paddle and lateralupper paddle. Additionally, the controller can be configured to apply adisplacement force to displace at least one of the medial upper paddleand lateral upper paddle relative to the lower paddle when engaging thefirst and second bones of the joint, or vary the displacement forcebased on flexion angle of the first and second bones of the joint orthroughout a range of motion of the joint. Further, the controller canbe configured to determine a gap spacing between at least one of themedial upper paddle and lateral upper paddle, and the lower paddle basedon the displacement force and a deflection factor, as further describedbelow. In any of the foregoing, the controller is configured to operateby the processor executing software or computer instructions stored inmemory to achieve the specified operation. Thus, the memory can havestored thereon e.g., a predetermined force profile for applying varyingdisplacement forces throughout a range of motion of the joint.

Force Sensor Calibration

The force sensors 130 are preferably pre-calibrated (factory calibrated)so that the device is ready to use in the OR. The sensors may be zeroedusing an adjustment process, for example a mechanical adjustment, toensure consistency and repeatability in the readings across devices.Alternatively, different calibration constants can be applied andassociated with each ligament balancer. The ligament balancer may havecomputer memory stored within the body 9 for storing a uniquecalibration file with calibration constants that are associated with thedistraction device.

Alternatively, the ligament balancer may be marked with a uniqueidentifier, such as an identifying (ID) number, or a radiofrequency (RF)ID tag that is inputted into the computer. In this latter case, thecomputer is equipped with an RFID receiver for receiving the RF signalfrom the RFID tag in the ligament balancer. The number may refer to alook up table stored in software on the computer that associates acalibration file with that number.

Alternatively, the calibration file can be provided on transportablememory media, such as a USB drive, flash drive, CD-ROM, or the like, andprovided with the ligament balancer. Thus when a ligament balancer isdeployed to a new site that already has a CAOS system, it can be shippedwith the accompanying calibration file that can be preloaded into thecomputer memory during installation of the device before it is firstused. Thus the calibration file need only be uploaded to the computeronce, upon which it can be used repeatedly. This makes servicing andrecalibrating the device easier.

Alternatively, the CAOS station may be equipped with an internetconnection (either wireless and/or wired) and the software with thecorresponding calibration file may be updated automatically via theinternet by wire or through the air. Alternatively, the calibrationconstants can be stored or coded in the ID number directly so that thelook up table does not need to be updated to include new calibrationfiles. Checks may also be included in the software to ensure the deviceID that corresponds to the device being used has been entered and acorresponding and valid ID file is present. The software may also beimplemented to monitor usage of the device (number of surgeries, timeused during each procedure, cycles endured, mean and peak forces exertedby and on the device). Checks can also be incorporated in the softwareto ensure the device is brought back for servicing when servicing isdue, or after reaching pre-established usage criteria (for example,number of surgeries).

Additionally, a separate calibration device or calibrator can beprovided to check and/or recalibrate the device in the OR. Thecalibrator may include a mechanical spring of known force-distancerelationship, such that when the ligament balancer is coupled with thecalibration device and the actuators of the ligament balancer areactivated, the spring will apply a known force based on the distance itis compressed or extended, and the distance can be measured using theligament balancer. Thus the force measurements made by the distractorduring actuation of the actuators can be compared with the known forcesapplied by the spring according to the distance travelled, and thesoftware can determine if the device is in or out of calibration, and ifrecalibration is required, and in the latter, can apply a newcalibration based of the measured and known forces during theintra-operative calibration process.

Additionally, the force sensors of the ligament balancer can becalibrated, or their calibration can be checked, by controlling andactuating the motors until they reach their limits of motion,essentially reaching a hard stop within the inside of the housing, andthen correlating the input power signal to the motors with the outputsignal of the force sensor. As an example of the process, the forcesensors in the distractor may be calibrated by the manufacturer afterassembly of the device by applying external known loads to the upperpaddles and then adjusting the force signal output by applying constants(in a linear or non-linear model) such that the force sensor output iscalibrated to the known applied loads. The distractor can then becommanded to go to its maximum height until it reaches the internalstop, where the linear motion of the sliders stops and the amount ofenergy going into the motor begins to increase, generating an increasingamount of load on the force sensors. The energy (electrical current)delivered to the motor can be increased until it reaches a specificvalue, and the relationship of the output signal of the force sensorrelative to the input energy signal to the motors can be quantified.Because the force sensor is initially calibrated using external loads orweights, the force corresponding to the energy or current inputted tothe motors can be determined. It is assumed that the relationshipbetween current inputted to the motors and force applied to the forcesensors when the sliders have reached the hard stop remains constantover multiple uses and over the service life of the distractor, andtherefor this relationship can be used to recalibrate the force sensorsduring use of the distractor, or to check the accuracy and reliabilityof the calibration of the force sensor either in the operating room orduring routine diagnostic tests before or after use. This may benecessary if the force sensors need to be recalibrated from time to timedue to repeated use, abuse (e.g., overloading causing damage or changesin the sensor characteristics), or repeated sterilization.

Modes of Control—Force, Height, Force-Height, Disabled, and Enabled

The CAOS system 1000 includes a user interface (see for example FIG. 9C)that allows a user to control the function and behavior of the ligamentbalancer 1. The user interface includes buttons for controlling theligament balancer in one of several modes, including a force controlmode, a height control mode, and force-height control mode.

Referring to FIG. 9B, in Force Control Mode, the user enters a targetforce for the medial and lateral sides of the displacement mechanism.The target force value may be the same or different for either sides.The actuators or drive assembly then actuate the upper medial andlateral paddles according to the error between the target force set bythe user and the actual force measured by the each force sensor (errorbased control loop). Thus the actuators will raise each upper paddle ifthe measured force is lower than the target force, until sufficienttension is applied to the ligaments such that the force applied by theligaments on the paddle approaches the target force. As the target forceis approached, the difference (or error) between the actual force andtarget force decreases and the rate of actuation decreasescorrespondingly, until the actual force equals the target force. If thetarget force is less than the actual force measured by the sensors, theactuator will move the upper paddle down reducing the tension in theligaments and soft-tissues surrounding the knee, until the actual forceapproaches and eventually reaches and stabilizes at the target force.

Referring to FIG. 9C, in the Height (or position) Control Mode, the userenters a target height, such as 10 mm, and actuator will move the upperpaddle up or down based on the current height until the target height isreached. The height dimension is referring to the distance between thelower surface of the lower paddle and the upper surface of the upperpaddle (or a minimum distance in the case of a concave-shaped upperpaddle surface). When an augment is used, the height dimension isreferring to the distance between the lower surface of the lower paddleand the upper surface of the augment (minimum distance in the case of aconcave-shaped augment surface). The additional height added by theaugment can be taken into account by the computer's software eitherautomatically depending on what step the surgeon is at in the surgicalprotocol (e.g., ligament balancing step with no augments, or virtualtrailing step with augments), or it can be taken into account manuallyby pressing a button on the user interface.

In the Height Control Mode, the distraction system is controlled basedon its height (or position), and displays the forces measured by theforce sensors that are acting on the upper and lower paddles. The userinterface for controlling the ligament balancer can be a graphical userinterface programmed into the software and displayed on the display,with buttons for control. The user interface can also include buttons108 (FIG. 6A) that are incorporated directly on the body of ligamentbalancer to allow a user to change the control mode, to set or adjust atargeted force or height value, or to start and stop or pause the motionof the ligament balancer. The buttons can be integral to (i.e., builtin) the body of the ligament balancer (for example, on the housing), orthey can be disposable and attached to the ligament balancer at thebeginning of the case. They can be clipped on to the body and made ofplastic to reduce costs, and can include a battery and wirelesscommunication with the computer and/or controllers.

The distraction device may also be switched to a disabled mode, wherethe actuators are not being driven or powered, and the user is able toback-drive the system by pushing down or pulling up on the upper paddlesor paddle connectors to manually change the height of the device. Whenthe device is disabled, the system is still able to read the position(height) of the upper paddles using the encoders or hall sensors. Whenthe device is enabled, it operates in one of the functional controlmodes.

In the Force-Height Control Mode, the distraction device 1 measures theforce-height (or force-elongation) relationship of the kneesoft-tissues, thus measuring the mechanical properties of thesoft-tissues surrounding the knee joint. The objective is to accuratelyand reliably characterize the mechanical properties of the kneesoft-tissue envelope, and produce plot or graph of the force (y) vs.displacement (x) curve for the medial and lateral compartments (FIG.5A). In this mode, the ligament balancer is inserted in the knee andstarting from a lower position, the upper medial and lateral paddleslift up and apply a progressively increasing amount of tension to thesoft-tissues. Thus the system is measuring the force and thedisplacement as the ligament balancer increasingly spaces apart the kneejoint. This measurement can be realized by any control modes, includinga 1) Constant Velocity Control (FIG. 5B) and 2) Force Velocity Control(FIG. 5C).

For Constant Velocity Control, the rate of displacement (velocity) ofeach actuator is controlled according to the current value of themeasured force (F), FIG. 5B. Velocity 1 (V1) is applied first, from thelowest position until Force 1 (F1) is reached. Then Velocity 2 (V2)which may be a lower rate, is applied until a maximum force (Fmax) isreached. Then, the actuator switches to disabled mode (see section belowon synchronization). All parameters may be adjustable by the user tosuit their preference for velocity and force characteristics. In asimpler mode, V1 and V2 can be set to the same velocity.

Synchronization—It is desirable to have both sides trace the upperportion of the force vs. displacement curve simultaneously, to minimizeany potential artefacts caused by tensioning only one side at a time(for instance, some soft-tissues such as the PCL could contribute to thestiffness curve on both the medial and the lateral side). Ideally eachside would be applying the same force on either side at the same pointin time. This can be approximated in the Constant Velocity Control modeas follows: 1) both sides start at V1 from a lowest position; 2) oneside will reach F1 first and will de-accelerate to zero velocity untilthe second side reaches F1; 3) then both sides will start at V2 untilFmax is reached; 4) the first side to reach Fmax will stop and wait(maintaining the force, or height) until the other side reaches Fmax;then both sides will become disabled. Alternatively, both sides maytrace the force-elongations curve at a constant velocity until a maximumforce is reached. One side will reach the maximum force before the otherside, and will deaccelerate and maintain the force (in force controlmode) or maintain the height, until the second side reaches the maximumforce, at which point both sides can either maintain the force orheight, switch to disabled mode, or lower their height.

Force-Velocity Control (FIG. 5C)—The force vs. gap curve can also beacquired using a simple error controller (such as aproportional-integral-derivative controller, or PID controller) wherethe motor velocity is proportional to the difference (error) between theactual force and the target maximum force. A Maximum Velocity (Vmax) andMaximum Force (Fmax) are set. The PID parameters may be determinedimperially (through tuning) to optimize the responsiveness, stability,and overshoot characteristics of the control loop, targeting a completesweep measurement in about 3-5 seconds. Once one motor reaches thetarget force Fmax, it holds its position until the second motor reachesFmax, then both motors are disabled (FIG. 5C).

The system may include analytical software to automatically analyze theforce displacement curve, for example, by finding linear or non-linearbest fit relationships for different portions of the force vs.displacement curve. A patient specific tension or force 276 or gap 277value may be determined based on the shape of the curve, eitherautomatically using linear or non-linear curve fitting techniques, ormanually by plotting the curve and allowing the surgeon to visualize theshape of the curve. For example, a typical force elongation curve may bedivided into several regions, including a first so called ‘linear’ partwhere the fibers of the ligaments are going from a non-aligned orcrimpled state 272 to an aligned state, and a so called second part 273,where the fibers are being tensioned within their elastic deformationregion, where the rate of increase of tension is directly proportionalto the rate of increase in elongation, by a constant stiffness factor. Athird portion of the curve may represent where fibers begin to fail andstart to become detached from the bone (plastic deformation).

The ideal patient specific tension 275 as determined from the force vs.displacement curve, may depend on the patient's profile andcharacteristics, such as the activity level of the patient, age, BMI,gender, and so on. For example, a surgeon operating on a young activepatient can chose to leave the knee joint in a higher amount of tension(i.e., further to the right of 275, along the curve 271 shown in FIG.5A) than in an elderly non-active and underweight patient (which may befurther to the left). Thus the present invention allows the surgeon toquantify the load displacement characteristics of a particular patient'sknee joint and then determine based on the patient profile where is theoptimal tension for that patient along the curve relative to distinctfeatures of the curve.

In accordance with another aspect, the present invention providesmethods and software executable for compensating for deflection of theupper and or lower paddles and/or displacement mechanism (and the motionof the motors relative to the displacement mechanism) under appliedloads. Forces applied to the upper arms of the ligament balancer when itis operated in height control mode, for example forces applied by thesurgeon during a knee stability test (e.g., varus valgus stress test) orby the knee ligament tensions, may cause deformation of the upperpaddles 22, 24 and/or lower paddle 12 and lower paddle attachmentinterface 16, and the actual height of the ligament balancer may notcorrespond to the targeted height of the ligament balancer. Similarly,forces applied by the ligament balancer in force control mode may causedeflection of the attachments and the height reported by the motors andcontrols may not correspond to the actual height. In order to correctfor these deflections, the relationship between the applied forces anddeflections may be measured and quantified under known loads a priori.This may be quantified as a deflection factor accounting for variousheights throughout the range or motion of each side, various anticipatedloads, and for each side and set of attachments (12, 22, 24). Thesemeasurements may be tabulated in a look-up table or an analyticalrelationship (for example linear or non-linear curve fitting) may beused to describe the load, height, deflection relationship.

During use, the applied forces measured by the force sensors may be usedto estimate the deflection of the ligament balancer at any given heightbased on the pre-established load-deflection relationship. In order tocompensate for deflection, when the ligament balancer is operated inheight control mode, the actual targeted height may be adjustedautomatically and in real time by the software running on the computerand or firmware running on the controllers (the position control loop)to compensate for the amount of deflection occurring, wherein the amountof deflection is known from the a priori determined load-deflectionrelationship.

For example, if 100N of applied force results in 1 mm of deflection onthe lateral side, when the ligament balancer is being operated in heightcontrol mode and is targeting an 11 mm thick insert, and as loads arebeing applied to the ligament balancer and the force sensors aremeasuring the loads, the targeted height is being adjusted according tothe measured loads, and when 100N is applied the height target isadjusted or increased by 1 mm. The height is increased proportionallymore or less when the measured force is more or less, respectively,according to the known load-deflection relationship. Similarly, theheights measured by the ligament balancer (via the motors, hall sensors,encoders, and/or controllers) in force control mode, or in theforce-elongation control mode, may be adjusted to account for thedeflection occurring under the applied loads, according to the knownload-deflection relationship.

Pre-Operative and Post-Operative Joint Kinematics

In accordance with another aspect, the present invention includescapabilities and methods for measuring the pre-operative (i.e., bonepre-resection) and post-operative (post-bone resection) kinematics ofthe joint. This is accomplished by tracking the relative 3D positions ofthe bones of the joint (tibia and femur) using the 3D tracking system 2and the reference markers 107, 106, associated with or attached to eachof the bones, and using software executable to compute multiple motionparameters that describe the relative motions of the bones.Pre-operative (pre-op) kinematic measurements are typically done afterthe initial registration of the knee joint anatomy, femoral 98 andtibial 99 mechanical axes, and bone coordinate systems. In order tomeasure knee kinematics, the surgeon takes the knee through a range ofmotion (flexion) and the 3D tracking system will measure the overallalignment of the mechanical axis (varus and valgus, as determined by theangle between the tibial and femoral mechanical axes) as a function ofthe knee flexion angle. The overall alignment of the mechanical axis maybe computed throughout flexion as the angle between the tibialmechanical axis and the sagittal plane of the femur, which is coincidentwith the femoral mechanical axis.

As shown in FIG. 9A, the user interface may include representations ofthe bones of the joint, showing the real-time relative positions of thebones 311. The pre-op kinematic measurements may also include thefollowing measurements: the knee flexion angle 312 including maximumextension and maximum flexion, which can be represented graphically on agraph 300, along with maximum varus angle and maximum valgus anglethroughout the range of flexion 303, medial 302 and lateral 301 gapvalues between the tibia and the femur throughout the range of flexion(average, minimum, and/or maximum), internal and external rotation ofthe tibia relative to the femur (average, range, or as a function offlexion). The medial 302 and lateral 301 gaps can be determined bycomputing a closest distance between the medial and lateral surfaces(respectively) of the femoral condyles and various points or planes onthe tibia or tibial insert, depending on the stage of the procedure. Forinstance, the medial gap can be computed by searching for the point onthe medial femoral condyle that is the closest distance to a specificpoint on the medial aspect tibia (such as a medial cut height referencepoint), or a plane on the tibia (for example, the planned or measuredtibial resection plane), where the closest distance could be searchedfor and computed along a particular direction, such as along the tibialmechanical axis direction, or along the direction normal to the plannedor measured resected surface.

As shown in FIG. 9B, in the case where a tibial resection has alreadybeen performed, the medial 324 and lateral 325 gaps can be determined bycomputing a closest distance between the medial and lateral surfaces(respectively) of the femoral condyles and the measured surface of thetibial cut. Pre-operative gaps can be computed as the distance between aplanned tibial cut surface, or between the tibial cut height referencepoints (tibial cut depths) digitized on the medial and lateral plateausof the tibia, and the medial and lateral surfaces of the native femoralbone.

As shown in FIGS. 10A and 10B, post-operative gaps 307 can be computedas the distance between the virtual tibial insert (for example, lowestpoint on the medial and lateral plateaus on the insert) and the virtualfemoral component (i.e., articulating surface of the implant).Post-operative gaps can also be computed as the distance between thevirtual tibial cut and the virtual femoral component (i.e., articulatingsurface of the implant). During the measurement of pre-op and post-opkinematics, stresses can be applied to the joint by the surgeon, such asvarus and valgus stresses at discrete flexion angles, or throughoutflexion, in order to measure the opening on the lateral and medial sidesof the knee, respectively. Graphical plots 300 or charts can begenerated on the display in real time to show the varus/valgusangulation, medial and lateral gaps, tibial rotation, and medial 345 andlateral 347 forces (FIG. 10A) plotted against the flexion angle. Pre-opand Post-op graphs can be shown side by side to allow for comparison ofthe pre-op and post-op kinematics.

Ligament Balancing

The CAOS system 1000 also includes capabilities for acquiring gap datawith the displacement mechanism 1 after a tibial resection has beenperformed but before any resections have been performed on the femur.FIG. 9B shows an example of a user interface that may be used to acquiregaps at different flexion angles or throughout flexion. The userinterface includes the display of the following values in real time:Overall varus/valgus alignment 320, flexion 321, the amount of appliedforce on the medial 322 and lateral 323 sides, the medial and lateralgap between the tibial resection and the native femoral condyle,displayed graphically on the bone models 324, 325, and as numericalvalues 326, 327, and on a graph 300 plotted against the flexion angle301, 302. The overall alignment 303 may also be displayed on the graph300, as real time (white dot) and maximum varus and maximum valgusangles, represented as the width of the bars 303 at each flexion angle.

The user interface may also include buttons 328, 329 for adjusting the(targeted) force applied by the ligament balancer on the medial andlateral side either up or down. Alternatively, the medial and lateralforce can set to the same value using only one button that adjusts boththe medial and lateral side at the same time. Color coded lines or bars311 may be used to indicate or highlight the amount of force beingapplied on the medial and lateral side, and may indicate the proximityof the actual force to the target force.

The ligament balancing step, where the positional relationship betweenthe tibia and femur is measured and stored throughout flexion, may beperformed in force control mode as shown in FIG. 9B, or it may be withthe ligament balancer operating in the height control mode, where themedial and lateral gap heights are controlled by the ligament balancerand buttons on the user interface, and the forces acting on the medialand lateral sides of the ligament balancer are displayed in real timeboth numerically and graphically on a chart as a function of flexion. Inheight control mode, the surgeon may adjust the height independently oneach side until the medial and lateral gaps between the femur and tibiaare filled up, using the force values to monitor the force beingapplied. The surgeon can then perform a stability test, such as a varusvalgus stability test to assess the stability of the joint with thatthickness of material being inserted in the knee. The on screen forceand gap values can be used to quantify and monitor the assessment andstandardize the force being applied.

In one embodiment of the present invention, the CAOS system 1000 may beused to assess how tight or loose the knee feels based on an appliedforce. The ligament balancer may be inserted in the knee after making atibial cut and operated in force control mode, tensioning the ligamentsand soft tissues until the targeted force is achieved. The surgeon maydecide to release ligaments until an appropriate (overall) alignment isachieved. Once the targeted force is achieved in force control mode andthe gap is stable, the ligament balancer may be locked at the currentheight. For example, by pressing a button on the screen that switchesthe operational mode of the ligament balancer, i.e., switching it from aconstant force mode to a constant height mode, FIG. 9C, so it is lockedat the height that created the targeted tensions and it displays theforces 322, 323 acting on the upper medial and lateral paddles in realtime. The surgeon can then stress the knee into varus and/or valgus, bypushing medially or laterally on the tibia or ankle, and whilemonitoring the opening of the gap 326, 327 on the lateral and medialsides. They can use the force readings (values 322, 323, or curves 349,359) on the display to control the force that they are applying on thetibia or ankle while monitoring the opening of the knee 326, 327 on theuser interface (navigation screen).

Thus a repeatable way of performing the stress tests from patient topatient as a result of the force readings is achieved. The goal ofevaluating the knee opening (gaps) is to allow the surgeon to feel howtight or loose the knee would be if the implants were planned to have azero gap value at the targeted applied force for that flexion angle, andto potentially determine if the knee is going to be too loose or tootight given the planned position before they make any femoral cuts, sothey can adjust the plan accordingly. This can be performed inextension, flexion, or any flexion angle. Gaps 301, 302, forces 349,359, and alignment 303 may be plotted against flexion (FIG. 9C), or theymay be plotted against time to show the correlation between the gaps,alignment, and applied forces as the knee is being stressed in varus andvalgus at a given flexion angle. Broken, weighted, and/or colored linesmay be used to distinguish the variables from one another and associatethem with the corresponding axis labels.

Applied Force as a Function of Flexion Angle

In accordance with another aspect, the CAOS system includes capabilitiesfor adjusting or controlling the function of the ligament balancer 1,including the amount of force being applied, or the height beingapplied, by the ligament balancer, as a function of the relativeposition of the bones of the joint. This has unique advantages, forinstance the amount of force being applied to the joint can beautomatically adjusted based on the current flexion angle of the tibiawith respect to the femur. This can be controlled in either a static ordynamic gap acquisition protocol.

For example, the system could be used to acquire gaps in the knee jointat specific flexion angles, such as at two or more of the followingflexion angles 0, 20, 30, 60, 90, 120 degrees of flexion. The specificdesired applied force at each of these flexion angles can be controlledby the computer's software, and can be inputted by a user before orduring each case, or can be stored in a user profile that includes theuser's preferences and preferred options. Thus, when the user insertsthe ligament balancer into the knee and brings the knee into extension,the ligament balancer can start from a lowest position in force controlmode and increase its height and applied force until the desired forceat around 0 degrees flexion (i.e., extension) is reached. Then, the CAOSsystem stores the relative position of the tibia and femur, and theassociated knee joint gaps. The user can then bring the knee intovarious degrees of flexion, while the software automatically monitorsthe knee flexion angle and actively controls the force being applied ateach flexion angle accordingly. The acquisition can be static, where theuser momentarily holds the knee at each flexion angle to acquire the gapvalue at that specific flexion angle, and the software automaticallyregisters the gap value once the user reaches the flexion value and theforce has stabilized at the target force at that flexion angle. This hasthe advantage of ensuring the target force has been achieved at eachtargeted flexion angle before the user moves to the next flexion angle,as well checking and ensuring other parameters, such as a neutralinternal or external rotation of the tibia with respect to the femurduring the acquisition.

Various criteria can be assigned for the automatic acquisition, such asa time criteria, wherein the flexion angle and applied force as measuredby the sensors has stabilized and are within a predefined value orthreshold, for example, stabilized for one or more seconds. Criteria forthe other parameters can also be assigned, such as force within acertain number of units (e.g., +/−5N), rotation with a certain amount ofdegrees, etc. An indicator can then be displayed on the screen informingthe user the gap at that flexion angle has been stored by the computer,signaling them to proceed to move the leg to the next flexion angle. Asthe knee is brought towards the next flexion angle the computer canbegin adjusting the force to approach the next desired force at thesubsequent flexion angle.

The acquisition can also be continuous (dynamic), wherein the systemacquires the gaps though out a range of knee motion (flexion), withoutpausing at each discrete gap value. In this case the 3D tracking systemcontroller is continuously monitoring the joint or flexion angle andinputting this into the controller, which is dynamically adjusting theapplied force as a function of the flexion angle in real time, while thecomputer is storing the relative position of the femur and tibia. Thusthe surgeon can dynamically move the leg throughout a range of flexionwhile the distractor controls the applied force and distracts the jointin real time, and the relative position of the two bones throughout therange of flexion and distraction is recorded and displayed to the user,and used to plan the position of the femur implant relative to the bone.

This has the advantage of being more efficient and acquiring continuousinformation that can be presented as curve over the continuum offlexion. In either the static or continuous acquisition scenario,kinematic (motion), dynamic (forces), and other parameters may bemonitored during the acquisition and displayed to the user in real time,such as internal-external tibial rotation relative to the femur,varus-valgus angle, medial and lateral gaps, anterior-posterior positionof the tibia with respect to the femur, femoral rollback on the tibia,and the load bearing axis (line joining the hip center to ankle center)and the position, such as mediolateral position, where it crosses theknee.

Virtual representation of the bone models may be displayed on the screenin multiple views or anatomical planes during the acquisition to providekinematic information and to help the user guide the motion of the boneduring the acquisition. The computer and controller may control theapplied force at intermediate flexion angles by interpolating betweenthe target values entered by the user for the specific flexion angles.The interpolation may be done beforehand once the user profile is loadedinto the software so the target force values may be pre-stored for everyflexion angle in a look-up table format. Alternatively, the target forcevalues may be calculated (or interpolated) in real time based on thenearest adjacent targeted force and flexion paired values. The user mayalso enter the desired applied force as a function of the flexion anglegraphically by drawing a line or curve on a graph of force vs flexion,or by editing points on the curve either with up/down buttons ordirectly on the curve itself by modifying node control points on thecurve directly. This can be done prior to or during the surgery. Thus auser is able to enter a desired profile for the applied force as afunction of flexion allowing the computer to automatically control theforce applied between the joint as the knee flexion angle is variedduring the procedure, this allowing the surgery and the implant positionto be planned with different tensions in the ligaments corresponding todifferent flexion angles.

For example, as illustrated in FIG. 7, a user may enter into their userprofile a greater applied force as a target in extension 160 (forinstance, 80N per medial and lateral side at 0 degrees extension), andlower force in flexion 161 (for instance, 50N per medial and lateralside at 90 degrees flexion). Additional target force values at otherflexion values can be entered as previously mentioned, and these targetvalues can then be interpolated, either linearly 162, or non-linearly,to achieve a target force for all possible flexion values. Limits forthe amount of flexion or the amount of applied force can be set by theuser, or automatically incorporated into the software. The adjustableapplied force vs. flexion graph can be displayed directly on theligament balancing user interface shown in FIG. 9B, thus allowing thesurgeon adjust the curve on the fly, or it can be established and storedin the user profile options prior to or at any time during a surgery.

In other words, the user interface shown in FIG. 9B can be configured toinclude a graphical display 333 (FIG. 9D) of the applied force vs.flexion profile. This graphical display serves to control and set theamount of force to be imparted on the joint. The graphical display ispreferably equipped with buttons 331, 332 to adjust the amount of forceselected e.g., at a selected flexion angle. The selected flexion anglecan be segmented into various intervals e.g., at every 10 degrees offlexion and configured with corresponding node control points 336 toallow for adjustment of the force setting.

Additionally, with the acquired force elongation profile or the appliedforce vs flexion profile, the user can plan resection depths based on apreferred or predetermined force to be imparted on the joint e.g.,between two joint bones of the knee joint. That is, the user selects thepreferred or predetermined force to impart on the joint and the computerdetermines the required resection depths for the tibial and femoralresections necessary to achieve the desired gap spacing corresponding tothe selected force based on the acquired force elongation profile.

Implant Planning with Predictive Gaps

The CAOS system 1000 has the ability to plan the position of theimplants based off the gap data acquired with the ligament balancer 1 inthe joint, using gap data that is acquired at various flexion angles.FIG. 8 shows an example of a graphical user interface 230 that may beused to plan the position of the femoral component based off predictedgap data. The interface includes representations of the femur 231 andtibia 232, which are derived from 3D bone models, and representations ofa femoral 233 and a tibial 234 implant positioned on the bones. The bonemodels are preferably generated by image free means, such as bydeforming a statistical shape model or atlas that is initially genericand not specific to the patient's bones as previously mentioned. Systemsand methods for creating computer bone models applicable to the presentinvention are disclosed e.g., in U.S. Pat. Nos. 8,126,533; 9,248,001;9,220,571; and 8,990,052, the entire disclosures of which areincorporated by reference herein for all purposes.

The interface allows for planning of either the tibia or femoral implantpositions, or both. The interface includes buttons 241 that allowadjustment of the position of the implant in any direction, such asVarus/valgus, Rotation, Flexion, Distal/proximal (Distal Resection),Anterior-Posterior (AP) Position, and Medial-Lateral (ML) position.Buttons may also be included for changing the type of implant (forexample, CR or Cruciate Retaining, PS or Posterior Stabilizing, UC orUltra-Congruent).

The CAOS system is equipped with executable software that calculates apredicted gap at various degrees of flexion 250. The predicted gap isthe amount of gap or space between the femoral and tibial implants whenthe implants are planned at their current locations, given the relativepositions of the femur and tibial bones as measured during the static ordynamic gap acquisition (ligament balancing) measurement. Thus thesystem has the ability to display a predicted medial 251 and lateral 255gap value or curve as a function of the flexion angle 256 of the kneejoint and as a function of the user's planned femoral and tibial implantpositions.

An advantage of this is that the user can observe the consequence of anychange in the implant position and the bone cuts on the predicted gap onboth the medal 251 and the lateral 255 sides throughout flexion,including in mid-flexion (for example, from 15 or 20 degrees to 60 or 70degrees flexion). Alternatively, the gap can be acquired and/orrepresented at several discrete flexion angles, such as 0, 30, 60, 90,120 degrees of flexion. The gaps may also be represented as the distancefrom the tibial resection to the femoral implant surface, and referencelines may be positioned on the graph to indicate the targeted tibialimplant (insert+baseplate) thickness (for example, a 10 mm thick implantwould be represented by a line at 10 mm gap throughout flexion) allowingthe surgeon to easily discern the difference between the predicted gapand the implant thickness throughout flexion, and whether there arespecific areas of flexion where the predicted gap is too tight (i.e.,less than 10 mm), or too lax (i.e., greater than 10 mm), and whetherthese discrepancies may be corrected by adjusting the position of theimplant. The predicted gap curves may also be color coded to highlightthe discrepancy between the targeted implant thickness (or targeted gap)and the predicted gap.

For instance, when the predicted gap is <1 mm than the planned implantthickness the curve may be color coded red to highlight potentialtightness in the corresponding area of flexion, when it is within −1 mmto 0 mm it may be color coded yellow, when it is within 0 mm to 1 mm or2 mm it may be green, and when it is >1 mm or >2 mm it may be blue, andso on. For instance, the predicted gaps may indicate that the knee isoverly lax in mid-flexion, and the surgeon may adjust the flexion of thefemoral component to change the shape of the predicted gap such that thegap is not overly lax in midflexion. Adjusting the flexion of theimplant may change the shape of the predicted curve depending on thedesign of the implant and the sagittal plane curvature (i.e., the radiiof curvature in the sagittal plane). For instance, femoral implantdesigns with so-called ‘J curves’ have different radii of curvaturealong different ranges of flexion, and so changing the flexion angle ofthe implant relative to the femur will change where the radii and outersurfaces are positioned on the bone and thus the predicted gap curves.

Thus a surgeon may be able to optimize ligament balance and gaps inextension, midflexion, flexion, or deep flexion by adjusting and finetuning the position of the implant to achieve the desired gaps or gapprofiles. Additionally, the predicted gap curve 250 may be presentedvertically with medial on one side and lateral on the other side asshown in FIGS. 9A and 9B, 300. Overall varus/valgus alignment data 303with maximum varus and valgus values (bars) may be superposed. Moreover,the overall alignment data may be predictive alignment data that isdependent on the angle of the implants or resections relative to thefemur and tibial bones.

To calculate the predicted alignment, it is assumed that the femoralimplant is articulating on the tibial implant on both the medial andlateral sides throughout flexion, so the overall predicted alignment isbased off where the femoral and tibial implants are positioned relativeto the mechanical axis of their respective bones (i.e., varus valgusangle and internal external rotation angle). The overall predictedalignment can thus be the angle between the tibial mechanical axis andthe femoral sagittal plane, assuming the medial-lateral axes of bothimplants are parallel and the femur is in continuous contact with thetibia. For instance, if a tibial cut is made at neutral relative to thetibial mechanical axis and a femoral implant is planned at 3 degreesvarus in the frontal plane, the overall predictive varus/valgusalignment shown on the screen during the femoral planning graph may be 3degrees varus. Thus the predictive overall alignment may be the sum ofthe individual femoral and tibial component alignments, which can becalculated throughout flexion and displayed or overlaid on the graph250. The pre-op alignment and gap curves may also be overlaid to allowfor comparison between pre-op and predicted post-op alignment and gapcurves.

The CAOS system also has the capability to simulate the looseness ortightness of the joint based on the planned positions of the implants.With the ligament balancer in the knee and the femur and tibial implantsplanned, the ligament balancer can be used in a height control modewhere it automatically adjusts its height according to the plannedposition of the implants and the degree of flexion that the knee ispositioned at. The height of the ligament balancer is set so that itreplicates the amount of implant that will be in the joint postresection of the bones, taking into account the difference in the nativebone geometry to be resected, and the planned implant surfaces andthickness. Any method for simulating the laxity of the joint may beused, including those described in U.S. Pat. No. 8,337,508, which ishereby incorporated by reference in its entirety for all purposes. Thusthe surgeon may use the predictive gaps shown on the display to evaluatepotential tightness or looseness, and/or they may use the actual kneewith the ligament balancer inserted and height controlled according tothe plan. The surgeon can perform a varus or valgus stress test at anyflexion angle, using the force readings on the display to control theforce (moment) that they are applying at the tibia or ankle andmonitoring the opening of the knee on the navigation screen. Thus theyhave a repeatable way of performing the stress tests from patient topatient and throughout flexion as a result of the force readings.

The goal of evaluating the knee opening (gaps) throughout flexion is toallow the surgeon to potentially determine if the knee is going to betoo loose or too tight given the planned position before they make anyfemoral cuts, so they can adjust the plan accordingly. This couldprevent femoral mal-rotation in flexion or flexion contractures/hyperextension. Advantages may include performing fewer recuts in the OR, orhaving to use a larger insert thickness and potentially elevating thejoint line.

The CAOS system also has the capability of providing predictive forcedata that is indicative of the amount of force acting in the joint andon the implant as a result of a specific implant plan. FIG. 10Cillustrates an exemplary implant planning user interface. The interfaceincludes predictive force information and data indicative of the forcesacting on the knee implant 503, 504, 505, 506, 500, 520. Based off theforce-elongation curve 271 measured in the knee with the ligamentbalancer, a relationship between measured force and elongation of thesoft tissues surrounding the knee (or relationship between force and therelative positions of the tibia and femur) is known on both the medialand lateral side at any particular flexion angle. The elongationmeasurement can be computed using either the motion controller of theligament balancer or the 3D tracking system. This relationship can thenbe then used to calculate and predicted forces as a function of theplanned implant sizes and locations in the bone, as well as the boneresection depths 500. The bone resection depths and angles determinewhere the implants will be positioned on the bone, and the thickness ofthe implant in each area determines how much material will be added orremoved in the joint in relation to the original native joint surface.It can be assumed that the tibial and femoral implants will articulate(be in contact) with one another once implanted, and thus the implantpositions in each bone will then define the relative positions of thebones after implantation. This relative bone positions post-implantationis used to determine the ligaments' new lengths at that position, or howmuch elongation the ligaments and soft-tissues undergo in comparison tothe pre-implantation state when the force versus elongation measurementwas taken. Thus the initially measured force versus elongationrelationship is used to determine the predict force which is dependenton the planned location and size of the implants and bone cuts.

The following is an example of how the force may be estimated. A forceelongation curve 271 (FIG. 5A) is acquired in extension on the medialand lateral side with the ligament balancer. The gaps measured for theforce elongation curve on the medial and lateral sides could be based ona distance between a fixed point on the femur and a fixed point on thetibia, for example the insertion point of the medial or lateralcollateral ligaments. Thus a reference position (or gap) between a tibiaand femur is defined both in 3D space and on the force vs. elongationcurve (for example 18 mm on the curve). There is a reference forceassociated with reference position, according the measured forceelongation curve.

Now, the tibial and femoral implants may be planned such that theirlocations relative to their respective bone is known, for instance, byplanning the depths of the resections, such as the depth of the distalfemoral resection on the medial (7 mm) and lateral (8 mm) side 242. Itis assumed that the tibia and femoral implant will be engaged orarticulating with one another such that there is contact between thefemur and tibia on the medial and lateral sides and the overall combinedthickness is known. For example, if the tibial implant is 10 mm thick507 at its lowest point on the plateau and the femoral implant is 9 mmthick at the thickest part of its distal aspect, i.e., perpendicular tothe distal resection, then the total combined thickness is 19 mm inextension. Now, based off of the planned location of the implants in thefemur and the tibia, and the total combined thickness of the femoral andtibial implants, a virtual gap value may be calculated (based off howmuch the implant thickness at its current cut location will spread apartthe joint) and compared with the known acquired reference position orgap. The difference between the known reference position and the virtualgap will determine the amount of elongation from the reference positionand hence the amount of force from the reference force (which isdetermined from the force elongation curve). Thus as the surgeon adjuststhe plan by increasing the insert thickness, or decreasing the depth ofbone resection on the tibia or the on femur, this will increase thevirtual gap values 503 and 504 and predict higher residual forces usingthe force-elongation curve 500. If too much bone is removed or theinsert thickness is too small, the predicted force may drop to zero orclose to zero as soon as the ligaments are no longer in tension.

The CAOS system also has the capability of predicting the force betweenthe tibia and femur implants throughout a range of joint motion, andparticularly through a range of flexion 520. A force-elongation curvecan be acquired in a single degree of flexion (for example inextension), and that force-elongation relationship can be applied atevery degree of flexion to calculate the predicted force on the medialside 521 and lateral side 522 throughout flexion. Alternatively, toimprove the accuracy of the prediction, multiple force elongation curvescan be acquired at different flexion angles, for example in extension(around 0 degrees flexion) and in flexion (around 90 degrees flexion)and the force-elongation data can be interpolated across the flexionangle on the medial and lateral side to calculate the predicted force atintermediate flexion angles between 0 and 90. Additional acquisitionscan be acquired in mid-flexion and to further improve accuracy and tohave additional points to interpolate between. Forces can also beextrapolated to hyper extension or deep flexion, and can include factorsto better predict the non-linear behavior at the extreme positions dueto biomechanical factors such as tensioning of the posterior capsule asthe knee is brought into hyper extension.

Once the surgeon has selected a suitable implant placement they canvalidate their plan and proceed to resect the bones to install theimplant components according to the validated plan. Validating the planincludes defining a set of targeted bone resections. In order to performthe resections, cutting guides may be navigated into the targeted cutpositions using the 3D tracking system. Alternatively, a robot 8, suchas a robotic cutting guide, or a robotic-assisted arm that guidescutting tools (burrs, saws, and the like), may be used to perform thecuts according to the plan. After the resections are performed, theposition of the final resection can be measured and stored using thetracked cut controller.

Input to Patient Specific Dynamic Model

The CAOS system can also include a dynamic biomechanical model of thepatient that can be used to predict the post-operative joint kinematicsand dynamics according certain surgical input parameters, such as theplanned position, alignment and size of the implant components, as wellas force-displacement data collected by the ligament balancer.Predicting the post-op joint kinematics means that the positionalrelationship between the tibia and femur (and optionally the patella),and their respective implants, is determined over a range of jointmotion, such as flexion. Predicted kinematic parameters over a range offlexion can include joint angles (e.g., varus/valgus, internal/externalrotation), femoral rollback on the tibia, and femoral condylar lift-offfrom the tibia. Predicting the post-operative joint dynamics means thatthe forces acting between the implant components, and optionally betweenthe implants and bones and in the surrounding soft-tissues, arepredicted. Inputs into the model can include the flexion angle, muscleloads, and anatomic structure geometries and properties. The dynamicbiomechanical model includes 3D geometry data of the bones, andinformation about the soft-tissues surrounding the joint, includingligaments (MCL, LCL, PCL), joint capsule, muscles (quadriceps,hamstrings) and tendons (patellar tendons). Soft-tissue information caninclude the attachment sites and lengths of ligaments, tendons, andmuscles, including geometry, volume, and cross-sectional areas. Themodel can be a dynamic model of a knee joint or entire leg or lowerskeleton that is capable of modelling or predicting the kinematics anddynamics of the knee joint during certain functional activities, such asa deep knee bend, stair climbing, and so on. The model can includeassumptions about soft-tissues characteristics (muscle activationforces, effective soft-tissue stiffness, Young's modules, visco-elasticproperties) that cannot be measured easily intra-operatively. Certainproperties can be determined from pre-operative data, such as image datataken from a pre-operative scan, such as a CT or MRI scan. Pre-operativedata can include static data such as measurement of the relativepositions of bones at a specific moment in time during variousactivities, such as standing, getting out of a chair, stair climbing,using imaging techniques such as 2D or 3D x-rays, ultrasound, etc. Themodel can simulate an activity such as a deep knee bend by simulatingmuscle loads such as the quadriceps and hamstrings which apply forces toeach bone at their attachments sites along the direction of the muscle.Contact loads can be calculated between the tibia and femur and femurand patella. Reaction forces such as ground reaction forces can bepredicted based on the patient overall mass, weight, height. Masses ofindividual body segments may be estimated using tables of known valuesbased off of measurements taken from a sample population of humans.Pre-operative dynamic data such as joint and body motion kinematics andgait analysis with ground reaction forces can be also be as inputs. Inthe present invention, the ligament balancer is inserted in the knee andrun through a displacement cycle from a lower to higher position andmeasures the displacement—force relationship of the soft tissuessurround the joint. This can be done individually for the medial andlateral side, or together on both sides. The force—displacementrelationship are then used as inputs to the knee model to moreaccurately predict the kinematic and dynamics of the knee based of aselected component placement, and thereby optimize the placement of thecomponent by selecting the placement that produces the most desirablekinematics and dynamics.

Asymmetric Functionality

The active ligament balancing system can also have asymmetric controlcharacteristics. For example, during certain modes of use, such asduring a varus valgus stress test, one of the femoral condyles may belifting off the articulating surface of the corresponding upper paddlewhen on the other side the condyle is in contact and applying a force onthe paddle. In this case it may be desirable to measure the gap on theopposing side of the joint (i.e., the side that is lifting off thepaddle), while maintaining a constant height on the opposite side. Thiscan be measured by the 3D tracking (CAOS) system, however, in some casesit is desirable to get the values directly from the distractor ratherthan from the CAOS system (for example, if the line of sight between the3D positioning measurement system (optical camera) and the bone trackersis obstructed or if the distractor is being operated in a stand-alonemode). In this case the distractor may have different control strategiesapplied to the left and right side, wherein the side that is measuringthe height is allowed to move upward with a given force that is highenough to maintain contact with the condyle but not large enough toapply significant tension to the ligaments. Thus the compressive forcebeing applied between the tibia and femur during a varus/valgus stresstest can be captured on one side of the distractor that is beingcontrolled to a constant height, while height of the oppositecompartment can be measured by controlling the distraction force of thedistractor.

Post-Resection Stability Assessment

The CAOS system also has the capability of assisting in the assessmentof the joint after the resections are performed. Here, the surgeon canuse the system to evaluate the residual tension in the joint fordifferent available thicknesses of the tibial insert. The ligamentbalancer can be assembled with the appropriate lower paddle 12, upperpaddles, 21, 23, and augments 42, 44 that match the size of the tibialbaseplate and tibial insert that is to be implanted. Once assembled, theligament balancer is inserted in the knee and used in height controlmode, where the height of the ligament balancer is controlled to matcheach of the available insert heights in the implant system to beinstalled.

FIG. 10A shows an example of a user interface that is displayed duringthe post-resection assessment. The interface includes frontal 343 andsagittal 344 views of the femur and tibia 3D bone models complete withresections and implants installed on them, and the position of the femurrelative to the tibia is displayed in real time according to the trackedposition of the femur and tibia. The locations of the implants on thebones can be determined either by their planned locations, or by thelocations of the measured resections, or by digitizing the implantdirectly. The interface also includes the real-time display of thedegree of overall alignment 320, degree of knee flexion 321, and medial326 and lateral 327 gap values, which may also be representedgraphically on the models in the frontal view 334, 335 and sagittalview. The amount of force acting on the medial 322 and lateral 323 sidesmay also be shown. A color coded bar 311, colored text, meter, or othergraphical objects may be included to highlight the magnitude of theforce being applied to the ligament balancer.

The interface may also include a graph 300 that plots out the overallalignment 303 (real time, mean, max varus and max valgus), and themedial 345 and lateral 346 forces being measured by the ligamentbalancer as a function of flexion. Medial and lateral gap values mayalso be included on the graph. Thus the surgeon can take the kneethrough a range of flexion and plot out and assess the forces acting onthe ligament balancer (which is now acting as a virtual implant orvirtual trial implant) as well as the joint kinematics throughout arange of flexion. The force curves 345, 346 may also be color coded todraw attention to the whether the measured force is relatively high orlow. Reference lines 347 may be included on the chart to depict theinitially targeted force during the ligament balancing steps to allowfor easy comparison. The color coding of the force curves could berelative to the initial force applied during the ligament balancingstage.

For example if a specific force profile was applied as a function offlexion, the difference between the measured force during thepost-resection assessment and the applied force during the ligamentbalancing step (before resecting the femur) could drive the color codingscheme, where values of higher force (for example 10N, 20N, 30N, 40N,50N higher) than the initially applied force are colored in progressiveshades of yellow, orange, and red in the color spectrum, to signify aknee that is increasing tighter than planned. Progressively lower forcevalues that signify less tension or a looser knee, may be similarlycolor coded, for example from green to blue.

Alternatively, color may be used to signify the amount of forceimbalance from medial to lateral at different degrees of flexion. Forinstance, if the medial force is greater than the lateral force by athreshold value (for example, 50N), or vice versa, the curves or valuesmay be highlighted to draw attention to the amount of imbalance. Theabsolute amount of force may also be used to set the color code (forexample, all forces >100N are orange, >150 red, and so on).Alternatively, the reference lines 347 and the color coding scheme canbe based on the predicted force that was predicted during the implantplanning stage of the procedure.

The gap values may also be color coded as previously described. Aninsert height button 340 can be used to change and control the height ofthe ligament balancer such that the height of the ligament balancermatches the height of the tibial insert being evaluated. Thus bypressing the insert button 340 on the screen or via the remote control,the surgeon can simulate different tibial implant thicknesses (or insertheights) and immediately evaluate the change in forces acting on theligament balancer due to the increasing or decreasing tension in theligaments, and based on the force, gap, and alignment measurementspresented on the user interface, can select the most appropriate tibialinsert thickness to use for this specific patient. Thus the ligamentbalancer can replicate and entire range of insert thicknesses and sizeswhile not requiring the large number of different sizes or thicknessesof components that are normally required in manual surgery.

The ligament balancer may also be controlled in increments of heightthat are finer than the available inserts thicknesses, for example in 1mm or 0.5 mm increments. Thus if the surgeon finds that an in-betweenthickness provides the best result, they may go back and further resectthe tibia by the difference between the preferred intermediate thicknessand the next available implant thickness to allow for the next size upof insert thickness to be used. For example, if tibial implants areavailable in 14 mm and 16 mm thicknesses, but the surgeon finds a 15 mmthickness provides the best result in terms of tension and stability,they can go back and recut the tibia by 1 mm, thus making room for the16 mm insert yet obtain the force characteristics of the 15 mm insert.

In other words, the height of the ligament balancer is controllable indiscrete increments of height, such as millimeters or incrementsthereof. The height of the ligament balancer can be set to a thicknessthat is in between the available tibial implant thicknesses of theimplant system. The foregoing allows for the next thicker size ofimplant from the height set to be selected and fitted to the patient byrecutting the tibia by the difference between the next thicker size andthe in between set height.

The surgeon can also use the ligament balancer and user interface toassess the post-resection stability of the joint, during for example avarus/valgus stress test. With the ligament balancer being controlled inheight mode, the surgeon may apply a varus stress and a valgus stress tothe tibia or ankle to evaluate the amount of opening in the gaps 326 and327 and change in the overall alignment 320 under the applied stress.The real time force values 322 and 323 can be used to control andstandardize the amount of stress (varus or valgus force) being appliedby the surgeon, as previously described, and this can be performed atdifferent angles of flexion and with different insert thicknesses.

If the surgeon finds that force being measured on the medial or lateralside is overly high while taking the leg through a neutral range offlexion (i.e., while not applying a varus or valgus stress), the surgeonmay use this information to perform releases on the ligaments or recutsof the bone and different angles or locations depending on the forceinformation being displayed (value, location, range of flexion).Performing a tibial or femoral recut at a slightly different angle (forexample 1 or 2 degrees) allows additional laxity to be introduced oneither the medial or lateral side (for example if more bone is removedmedially or laterally, respectively), or the tibia may be resected withmore slope to increase the flexion gap, or less slope to increase theextension gap. Alternatively the distal femur may be recut to increasethe extension gap and gain more extension of the leg when there is anextension deficit.

The ligament balancer may also be operated in a force control modeduring the post-resection assessment step, by pressing a button on thescreen that switches the ligament balancer from the constant height tothe constant force mode. As shown in FIG. 10B, tension 322, 323 isactively applied to the medial and lateral side of the joint accordingto the targeted force entered using the onscreen buttons 306, or storedin the user's profile of preferred settings, and the medial and lateralgaps between the femur and tibia 307, 309, can be monitored in real timeand plotted 350, 351 against flexion. If the gap (tibial bone cut tofemoral implant gap, or tibial implant to femoral implant gap) is toosmall relative to the desired gap, ligament releases can be performed toopen up and achieve the desired gap. Soft tissues may be releasedprogressively in preferred sequences, while monitoring the change in gap(force mode) or change in force (height mode) in real time, therebyhelping to reduce the risk of over-releasing a structure. Needles, smallscalpel blades, and the like can be used to more precisely control therelease process.

The system is also has the ability of estimating or determining thelocation of the contact force acting between the bearing surfaces of theimplant, based on the location of the femoral component relative to thetibial component as measured by the 3D tracking system. Collision orcontact detection software or similar algorithms can be used to detectwhere the femoral implant model is in contact with the tibial implantmodel. The implant model files are typically 3D geometrical mesh modelswith vertices and edges arranged as facets to form a solid or surfacemodel.

One method of determining the contact point is to search for a point orarea of intersection between the two models. The algorithm can alsosearch for zones of overlap between the femoral and tibial implantmodels and defer based on the degree or shape of the area or volume ofoverlap the location of a contact point or contact area between thetibial and femoral implants. The shortest distance between points on thesurface of the tibial model to points on the surface of the femoralmodel (i.e., a closest point algorithm) may also be used. Knowing thematerial properties of the implants (such as Young's modulus), themeasured forces, and contact locations or areas, the amount of stressacting on the tibial insert may also be calculated and estimated.

A finite element model may also be used to calculate the stresses actingon the tibial insert based on the measured loads and contact areas orpatterns. The user interface may include a top view of the tibialimplant (i.e., view aligned with the proximal-distal direction) tobetter illustrate the location of the force or forces (or contactpoints) on the tibial implant plateaus or articulating contact surfaces,as well as the predicted stresses acting on the implant surface. Thusthe surgeon may assess the contact pattern, contact forces, and/orcontact stresses of the femur on the tibia as they are bringing the kneethroughout a range of flexion.

Thus the CAOS system allows the user to visualize the contact patternsand they may look for specific contact or loading patterns that areindicative of a normal or favorable kinematic or dynamic outcome, suchas a pattern depicting the femur rolling back on the tibia with flexion,where the femoral-tibial contact point translates posteriorly on thetibia as the knee is flexed, particularly on the lateral side orproportionally greater on the lateral side than the medial side.

Alternatively, they may visualize paradoxical motion such as anteriortranslation of the femur on the tibia with increasing knee flexion, andmay decide to make a change to the position of the implants, or releasecertain soft tissues as a result. Changes may include adjusting therotation and/or position of the tibia on the tibial cut, byrepositioning the ligament balancer on the tibial cut, andre-evaluating. Thus the system may also be used to optimize or adjustimplant positioning during the post-resection assessment (or virtualtrialing) phase.

As previously mentioned, the CAOS system may be used to determine theoptimal rotation and/or position of the tibial component. For example,after the femoral component has been inserted and the surgeon isperforming a post-resection assessment, the load imbalance between themedial and lateral side during a flexion motion may be evaluated and ifan imbalance is detected the position (rotation, or AP or ML position)of the ligament balancer on the tibial resection can be changed and theassessment re-performed to see if the force imbalance improves.

Additionally, the location of the contact areas or points of the femuron the tibia can be used to determine if the tibial baseplate needs tobe repositioned. For example, if the contact points do not remain nearthe bottom of the dishes of the tibial implant but ride up the dishesand to one side as the knee is brought in to extension, it may be due toa suboptimal rotational position of the tibial insert with respect tothe tibial cut and femoral implant (tibial-femoral mismatch). In anotherembodiment, the ligament balancer may be configured to facilitaterotation and/or sliding of the lower paddle on the tibial cut surface.The bottom surface 13 of the lower paddle 12 may be adapted to rotateand or slide on the tibial cut surface, by for example incorporating alow-friction surface, such as a polished surface.

Alternatively, as shown in FIG. 12A, an intermediate part 370, that actsas a bushing, or a sliding surface for the lower paddle 12, may beplaced on the cut surface 110. The intermediate part 370 has alow-friction surface 371 for the bottom surface 13 of the lower paddle12 to more effortlessly rotate and/or slide on. The bearing surface mayinclude a feature, such as a cylinder 372, that mates with a feature onthe lower paddle, such as a central hole 373, to constrain the rotationof the lower paddle about an axis. The thickness of the bushing orintermediate part 370 may be compensated for by making the lower paddlethinner, or by adjusting the height between the upper and lower paddlesof the ligament balancer automatically via the computer's software.

FIGS. 12B-D show another example of how the ligament balancer may beconfigured to rotate on the tibial cut surface. In FIG. 12B, the lowerpaddle 12 is initially positioned on tibial cut surface 110. A rotatingbushing 362 that is intended to allow rotation of the lower paddle 12relative to the tibia is coupled to the lower paddle 12. The rotatingbushing 362 may have a distal portion 363 that is intended to beinserted into the bone, and permit rotation of the bushing and the lowerpaddle relative to the bone via a cylindrical surface. A drill or punchmay be used to create a cavity in the bone for the bushing, and thecavity may coincide with the final cavity that will be created for thetibial stem or keel. A drill or punch guide may positioned on the lowerpaddle and be used to guide the drill so the cavity created coincideswith the bushing.

As shown in FIG. 12D, once the rotating bushing is attached, theligament balancer may be attached to the lower paddle 12 via theattachment screw 19. The ligament balancer can be operated in forcecontrol mode, where it is applying a force to the femur and during aknee flexion motion, the balancer rotates and/or slides under theanterior and posterior and/or sheer forces imposed by the femoral trialcomponent or implant on the augments 42, 44 of the ligament balancer,causing the balancer to find a preferred position on the tibia as aresult of the loads. The ligament balancer can be also operated inheight control mode, and during a knee flexion motion, the balancerrotates and/or slides under the sheer forces imposed by the femoraltrial component caused by the tension of the ligaments, causing thebalancer to find a preferred position on the tibia. Alternatively, thebalancer may stay in position on the tibial cut but may be manuallymoved or rotated on the cut based on the observed measurements anditeratively positioned and re-evaluated until the surgeon is satisfiedwith the displayed measurements. Once the preferred position on thetibia is found, the position of the lower paddle may be marked on thetibia, for example using a surgical ink marker. The lower paddle may befixed, and the cavity for the tibial keel can be created to fix thefinal position of the implant. Alternatively, as previously mentioned,the position of the ligament balancer may be tracked by attaching areference marker and tracking it's position during the range of motion,and it's final preferred position stored in the computer. The range ofmotion or rotation of ligament balancer with respect to the tibia andfemur can be tracked and displayed, and the final preferred position maybe determined from the tracked motion pattern, for example in the middleof the extreme ranges of internal and external tibial rotation duringthe range of motion.

EXAMPLES

Several scenarios of how the CAOS system can be used clinically aredescribed below.

A) FIG. 13 shows a process flow chart overview 400 of how the CAOSsystem may be used in a tibial cut first ligament balancing technique(dependent cuts, navigated).

401 Set-up instruments: Assemble and calibrate instruments, assembleligament balancer 1 with appropriate size and side (left or right) ofupper 21, 23 and lower 12 paddles, and home the ligament balancer (notethe ligament balancer may also be assembled and/or homed later in theprocedure as described below).

402 Expose knee joint, remove osteophytes.

403 Attach a reference markers 106, 107 to the femur 100 and to thetibia 105 to permit tracking of the femur and tibia with the 3D trackingsystem 2.

404 Register anatomy of proximal tibia 105 and distal femur 100 and oftibial mechanical axis and femoral mechanical axis, creating a model ofthe tibial bone and model of the femoral bone. As previously mentioned,the models are preferable created using image free means, such as bydeforming one or more generic bone models to the points acquired on thebone surface. However any means for creating and registering a model maybe used, including image-based means that use models derived frompre-operative images such as CT, MRI or X-rays.

405 Measure pre-operative kinematics of the leg by taking the kneethrough a range of motion and measuring by displaying in real time andstoring the overall alignment of the mechanical axis (varus and valgus)as a function of the knee flexion angle (FIG. 9A). The pre-op kinematicmeasurements may also include the following measurements: the kneeflexion angles including maximum extension and maximum flexion, maximumvarus angle and maximum valgus angle and medial and lateral gap valuesbetween the tibia and the femur throughout the range of flexion or atspecific flexion angles, while applying a varus stress and a valgusstress, throughout the range of flexion (average, minimum, and/ormaximum), internal/external rotation of the femur with respect to thetibia. Preliminary releases may be conducted at this stage to address asignificant deformity.

406 a Plan tibial resection using tibial bone model and/or acquiredpoints on the tibia. The tibial resection planning parameters includemedial and lateral cut depths, cut slope angle, cut varus/valgus angle.Planning may also include internal/external rotation, medial/lateral andanterior-posterior positioning. Note this tibial planning step may beomitted and the user can proceed directly to using the displayedreal-time navigation values of the position of the tibial cutting guiderelative to the tibia.

406 a Track position of the tibial cutting guide relative to the tibialbone to achieve targeted (planned) position, fix position of cuttingguide relative to the tibial bone. Adjust (fine tune) position to moreclosely match target if using an adjustable cutting guide. Note arobotically positioned cutting guide may also be used.

406 a Perform tibial resection using positioned cutting guide, removeresected proximal plateau of tibia, remove guide.

407 a Measure the 3D location and angle (cutting depth, slope,varus/valgus) of the tibial cut with respect to tibia using the cutcontroller probe and store in computer.

Size tibia using array of lower paddles as templates. The cut surface ofthe tibia or the cut surface of the removed proximal plateau of thetibia can be used to determine the best size by overlaying one or moreof the different sizes of lower paddles 12. The appropriate size can beattached to the ligament balancer 1 using the attachment interface 16.Alternatively, the size can be determined from the tibial bone computermodel, which may be the same model that is used to plan the tibialresection, or the points acquired on the tibial bone surface. Thefemoral bone model can also be used to determine the most appropriatesize of upper paddles to be used. This can be accomplished byautomatically measuring on the model the medial-lateral size of thefemur in the vicinity of the articulating surface of the femur and/ortibia, for example using the medial lateral distance or absolutedistance between the most distal points on the medial and lateralfemoral condyles, or between the most posterior points on the medial andlateral femoral condyles, or both. Similarly, the distance between themost prominent points on the medial and the lateral condyles (apexes ofthe condylar surfaces) from an extension position to a flexion position(for example between 0 and 90 degrees of flexion) can be calculated(i.e., the areas of the condyles that would contact the contact surfaces22, 24, of the upper paddles 21, 23).

Alternately, the femur or tibia bone models may be initially sized witha femoral or tibial implant by the computer and the determined implantsizes can be used to determine which size of upper or lower paddles toattach to the ligament balancer. This allows the surgeon assistant whois assembling the ligament balancer on the back table to know preciselywhat size of attachments to use before passing the ligament balancer tothe surgeon, thus not requiring a pre-operative image to determine themost appropriate sizes of the attachments. The ligament balancer may behomed at this stage if it has not already been homed.

408 Insert the ligament balancer, which is preferably in a retractedposition and may be in a disabled or back-drivable state, in the kneesuch that the lower surface 13 of the lower paddle 12 rests on theresected surface 110 of the tibia 105, the rotation of the ligamentbalancer and the lower paddle can be set on the tibial resection suchthat the lower paddle provides a good anatomical fit to the tibia, i.e.the size and rotation of the lower paddle (the contour of the lowerpaddle matches the contour of the tibial implant baseplate) is set sothat it closely matches the contour of the tibial resection. Therotation of the ligament balancer can also be established later in theprocedure, (i.e., after planning and resecting the femur), and thepositioning of the ligament balancer on the tibial resection at thisstage can be done just in a preliminary manner such that it is in anapproximate position and the upper paddles do not interfere with thepatellar tendon and other soft tissues during the gap acquisitionsthroughout the range of flexion.

409 Gap acquisition under force control: With the leg in extension,start the ligament balancer in force control mode by pressing a buttonon the display or remote control (for example, by pressing a startbutton 108 directly on the ligament balancer, a go forward or engagebutton 330 on the display user interface (FIG. 9B), or holding down abutton on the footswitch), to apply a constant force in the knee jointon the medial and lateral side. The amount of force initially set can bebased on the settings programmed into the surgeon's user profile, andthe values may be different for extension and flexion. The appliedforces may also be adjusted in real time using the buttons 329 on thedisplay or remote control (e.g., 50-200N targeted per side, adjustablein increments of 5 or 10N). Upon pressing start the upper paddles willmove up and away from the lower paddle and begin to apply a forcebetween the tibia and femur. Once the targeted force has been reached.

410 is the stage the surgeon can measure the initial limb alignment 320and assess if the alignment and other parameters are acceptable. If theydetermine it is not acceptable, they may perform soft tissue andligament releases 411 as required or desired by the surgeon to bring thelimb into neutral mechanical axis alignment or with an acceptable rangeof parameters, for example, within +/−2 degrees of neutral overallalignment 320.

For instance, if the limb is in a varus alignment in extension of morethan two or three degrees, the following structures can be progressivelyreleased until the HipKneeAnkle (HKA) angle 320 is within 2 degrees:Step 1—release of pes anserinus, step 2—release of the deep later of themedial collateral ligament (MCL), step 3—release of superficial layer ofMCL, step 4—release of semimembranous tendon. For a valgus deformity,the following soft tissue structures may be released: step 1—iliotibialband, step 2—lateral retinaculum, step 3—LCL from the inside out.

To facilitate a controlled release, a small scalpel blade (such as a no.15 blade) or preferably a needle and a puncture technique can be used topuncture individual or small bundles of fibers at a time, whilemonitoring the alignment values in real-time on the display. The needleor scalpel can be inserted between the upper and lower plates to obtainaccess to the inner medial and lateral side of the knee, for example inthe case of a varus or valgus knee, respectively. The distractor willcontinue to apply a constant force and as the soft-tissues are releasedthe gap will increase, and the surgeon can monitor the increase in thegap as well as the change in overall mechanical alignment of the limbunder the force being applied by the ligament balancer. Once anacceptable alignment is reached under the constant applied force, therelative position of femur and tibia in extension is measured by the 3Dtracking system and stored in the computer.

409 The surgeon can now reacquire the relative position of the tibia andfemur and calculate the gaps between the bones dynamically in extensionand/or throughout a range of flexion in force control mode (FIG. 9B)after performing releases. Two gap curves can be generated, one for themedial 301 and one for the lateral 302 compartments.

412 Plan femoral component using the kinematic gap data measured bynavigation system in constant force mode 409. The implant gap may beevaluated at several different flexion angles, and the femoral componentcould be positioned to have a constant gap throughout flexion, or atzero and 90 degrees. Note at this stage if the surgeon cannot find anacceptable compromise between alignment and balance/gaps they can decideto perform additional releases 411, or to recut the tibia, for exampleto increase or decrease the tibial slope to change the flexion andextension gaps, and the re-assess the tibiofemoral kinematics 409 untilan acceptable trade-off is obtained.

413 Once the femoral plan is validated the femoral resections can beperformed, using either a robotic cutting guide or manual cuttingblocks. The femoral cut surfaces may also be measured with the cutcontroller, such as the distal cut, anterior cut, and/or all cuts, andtheir positions stored into the computer.

414 the femoral trial component is then inserted on the prepared femur.

415 the ligament balancer is then assembled with the appropriateaugments that match the tibia insert and/or femur to be implanted. Notethe augment may be selected to match and articulate with the femoralcomponent, and the implant system may have several available tibialinsert sizes that match a specific combination of femoral components andtibial baseplate components. For example, some tibial baseplates willaccept one size (the corresponding size) of tibial insert, plus one sizeup and one size down to allow for matching of the femoral component whenthere is a mismatch of up to one size between the femur and tibia. Othertibial baseplates are designed to accept any size of tibial insert sothat the tibia insert always matches the femur and any size of femur maybe selected to fit the femoral anatomy of the patient (see for examplethe OMNI APEX Knee system by OMNIlife science, Inc. of East Taunton,Mass.). Other tibial baseplates are compatible with only one size ofinsert, that size of insert is designed to articulate with multiplesizes of femoral components. At this stage the appropriate lower paddlesize can be determined and attached to the ligament balancer if not doneso already.

416 The post resection stability assessment (FIGS. 10A and 10B) is thenperformed. After inserting the ligament balancer (if it was previouslyremoved), the surgeon can evaluate a given tibial insert thickness andassess the tension (forces 345, 347, 322, 323) acting on the insert andthe corresponding knee gaps 326, 327 throughout flexion. The ligamentbalancer is preferably in height control mode and its height isautomatically set to match the insert thickness according to the plan.The ligament balancer may start from a low position to facilitateinsertion in the knee, such as the thinnest available insert thickness,or a lowest possible position. Different insert thicknesses may besimulated and evaluated by adjusting the insert height with the onscreenbuttons 340. The surgeon can directly visualize the change inforces/tension acting on the insert for different insert heights, andthey can apply varus and valgus stresses using the forces to control theapplied force and assess the opening of the knee throughout flexion. Thebalance may also be assessed in force control mode (FIG. 10B), where theligament balancer is applying a constant or programmed force profilethroughout flexion and the surgeon evaluates the gaps 309, 352, 353 toachieve the desired gap opening throughout flexion.

The computer or controller, which includes a memory, can also beconfigured to include a predetermined force profile for having thedistraction device apply varying displacement forces throughout a rangeof motion of the joint. That is the force applied by the distractiondevice can be configured to vary based on flection angle of the joint.Further, the memory can have stored thereon various predefined forceprofiles and user preferences for said force profiles.

417 The surgeon may assess whether the kinematic and gap parameters(final alignment, forces, gaps, contact patterns) are acceptable and ifnot, they may choose to perform soft-tissue releases or adjust the boneresections 418. Ligaments may be further released depending on the areaof tightness and residual deformity, and/or the implant positions andbone resections may be re-planned and re-performed to correct animbalance. For instance if the forces measured in extension are too highor the knee cannot be brought to full extension, a distal femoral recutmay be performed and the femoral implant may be elevated proximally. Thefemoral component may be downsized to accommodate a tight flexion gap,or the slope of the tibial component may be adjusted and recut. Ligamentreleases can be performed according to any sequence. The rotation of thetibial component can be assessed by observing the position of the lowerpaddle on the tibial bone cut and by monitoring the resulting forces,gaps and/or contact patterns during the assessment for any givenrotation. The rotation of the ligament balancer with respect to thetibia can be adjusted (with or without the use of an intermediate part370 or bushing 362) and the results reassessed.

419 Once the surgeon is satisfied with the measured parameters andresults they can proceed to punch the tibia, creating a cavity for thekeel of the tibial implant as previously described and as shown in FIG.11C. If the femoral component requires additional preparation, such asdrilling of lug holes via the trial, then this may also be performed.

420 The final implants can be inserted with or without cement, dependingon the preferred technique. If cement is used, the ligament balancer maybe inserted in the knee in place of the tibial insert and used tocontrol the forces during cementation of the components.

421 A final post-op assessment may be performed with the final implantsin place (this may also be performed in step 416) and stored in thecomputer. The final assessment 421 or the post-resection assessment 416may be compared to the pre-op kinematics 405 or 409, via side by sidecharts or overlaid charts.

422 The final case report is saved and includes all measured parameters,and can be printed or exported to an external storage medium, such as aUSB key, emailed to the surgeon, or sent to the hospital network andintegrated in the electronic patient record or stored in cloudrepository or registry.

Several variations to the above method can be envisioned, including:

Measuring pre-operative gaps and kinematics for a limited subset offlexion angles only, such as at 0 and 90 degrees of flexion.

Lock the height ligament balancer while applying a constant force inextension and/or flexion and assess the stability and feel of the kneeat those flexion angles with the height set. Here the height of theligament balancer can be increased or decreased (339, FIG. 9C) to assessjoint stability according to a gap that is planned with a tighter(smaller) or larger (looser) gap accordingly.

A force-elongation acquisition can be performed before, after, or duringthe ligament balancing phase, and the curves can be used to plan aspecific patient tension 275 and that tension can be used to plan theposition of the components to achieve the optimal tension. The planningscreen could include predicted force or tension curves based off theforce elongation measurements and the surgeon can see the effect ofcomponent position on the predicted forces/tensions and the gaps. Theapplied force during the ligament balancing step may be a constant forceor a programmed profile where the applied force varies according to theflexion angle.

In sum, the predicted force as a function of the planned gap can bedisplayed, i.e., measure force elongation (gap) relationship, prep andsize using resection information (depth of cut, tangency to bone, etc.),display predicted force (or ligament tension/forces or soft tissuetension/forces) as a function of the planned gap and measuredforce-elongation relationship. For example, the planning screen caninclude a display (FIGS. 9B and 9D) of a predicted force on the implantor implant model (e.g., a femoral or tibial implant) as a function ofgap spacing, i.e., the spacing between the bones of the joint or flexionangle of the joint, based on the measure force elongation profiles ofthe joint.

As illustrated in FIG. 13B, another variation of the above method isillustrated. The method 450 includes performing the femoral implantplanning and bone resection steps first 406 b, followed by the tibialplanning and resections 406 b, and then to use the ligament balancer forassessing tension and balancing after the resections have been made. Inthis case, femoral and tibial implant planning may be performed insequence (i.e., one after another), or simultaneously before proceedingto their respective bone resections, so that the total amount of bonebeing removed from the tibia and femur may be planned and evaluated. Theimplant planning may be based on bone anatomy data (measured resectionsor cut depths and angles with respect to bone anatomical data), as wellas pre-op kinematic data (included predicted gaps) acquired beforeresections are performed 405.

Femur First Ligament Balancing

In accordance with another aspect of the present invention, theorthopedic distraction device 1 is configured to allow the user to planthe position of the implants using both joint gap and bone resectiondata before making any resections on the bones. In this case, theligament balancer 1 is configured to distract apart the joint under acontrollable load and measure the relative displacement of the jointbefore any resections are made in the joint.

In another exemplary embodiment, the distractor has upper and lower armsor paddles that are configured to be inserted into the joint prior tomaking any resections. Referring to FIG. 14A, a frontal view of a femur100 and tibia 105 in extension is shown, with an upper lateral paddle205 and lower lateral paddle 201 and upper medial paddle 206 and lowermedial paddle 202 inserted in between the uncut tibia and uncut femur.The upper and lower paddles are thin enough such that the minimumcombined height of the upper and lower paddles (i.e., when the upperpaddle is in the lowest position) is small or thin enough (on the orderof 1-4 mm) to allow the device to be inserted into the joint beforeresections.

FIG. 14B illustrates a different possible arrangement of the paddles inwhich the upper medial paddle is positioned adjacent to the lower medialpaddle in the medial-lateral direction. A similar paddle arrangement isshown on the lateral side. This allows the minimum overall height of theupper and lower paddles to be smaller than that shown in FIG. 14A sincethere is additional clearance for the upper paddle to be brought down toa lower position (i.e., without interfering with the lower paddle).Additionally, as shown in FIG. 14C, the upper 205′, 206′ and lower 201′,202′ paddles can have several individual ‘struts’ or tines (like tinesof a fork) i.e., a plurality of struts that inter-lie adjacent to oneanother to maximize the overall surface area where the femur and tibiacontact the upper and lower paddles respectively. The arms/paddles mayalso have a slender and curved profile to allow the distraction with thepatella of the knee reduced in the grove of the femur. Although FIGS.14A and 14B show the knee in extension, the same arrange can be used tomeasure the gap on the medial and lateral side at any flexion angle,including at 90 degrees of flexion, and through a dynamic range ofmotion.

FIG. 15 shows another embodiment of the orthopedic distraction devicethat allows distraction of at least two bones of a joint before making aresection on either side of the joint. Here, the distractor device isfixed to a bone on one side of the joint, for example the tibia 105 of aknee joint, with extra-articular fixation means, such as one or morepins or screws 210. The fixations means may also include a coupling part211 that allows the displacement mechanism 5 of the distractor to attachto the pins or screws. The coupling part 211 may include a quickcoupling mechanism that allows the distractor to attach to the pinsquickly and preferably without requiring tools such as a screw driver.The coupling part may be attached to the pins and may have holes forguiding the insertion of the pins. The pins may also be used to supportcutting guides for making resections on the bone, for example after theplanning of the cuts using the gap data obtained with the distractor. Inparticular, an adjustable cutting guide wherein the guiding portion ofthe cutting guide is adjustable relative to a base that is attached tothe bone with pins or screws 210 (such as the Nanoblock product marketedby OMNI), can be used. Here, the adjustable cutting guide is attached tothe coupling part 211 and used to make the tibial resection.

The graphical user interface 230 shown in FIG. 8 may also be used tosimultaneously plan the position of the femoral and tibial componentsbased off predicted gap data. The interface includes representations ofthe femur 231 and tibia 232, and buttons 241 that allow adjustment ofthe position of the implant in any direction. Separate buttons can beincluded adjusting the femur and tibia positions and sizes.Alternatively, a femur/tibia button 243 can be used to toggle betweenthe femur and tibia, allowing the same buttons to be used for both thefemur and tibia (i.e. when one of ‘femur’ or ‘tibia’ is selected usingbutton 243, pressing the appropriate buttons will change the plannedvirtual position of the femoral component on the femur or tibialcomponent on the tibia, respectively). When both the femur and tibia areplanned on the same interface, the user can directly see the totalamount of bone being resected on either side of the joint, therebyevaluating the total amount of bone that will be removed from anycompartment of the joint.

As previously described, the predicted gap 250 is the amount of gap orspace between the virtual femoral and virtual tibial implants when theimplants are planned at their current locations, given the relativepositions of the femur and tibial bones as measured during the static ordynamic gap acquisition measurement. Since the static or dynamicacquisitions can be acquired prior to resection of the tibia, the systemhas the ability to display a predicted medial and lateral gap value as afunction of the flexion angle of the knee joint (250, 251, 255) and as afunction of the user's planned femoral and tibial implant positionsbefore any resections are made. Thus when changing the position of thefemur or tibia on the bone, the user can directly see the effect thatthese changes have on the predicted medial and lateral gaps. This hasthe advantage of allowing a user to carry out the femoral resectionsprior to the tibial resection, while still basing the plan off of theknee gap data.

As an example, the pre-resection gap acquisition and implant planningprocess may carried out as follows:

Attach reference markers to tibia and femur.

Acquire patient boney anatomy using navigation system (mechanical legaxis, bone morphing/mapping of the exposed anatomical areas of thejoint).

Establish a force with which to distract the bones apart and enter intothe computer via the control interface.

Insert ligament balancer and acquire the kinematics of the femurrelative to the tibia at various degrees of flexion while the distractoris simultaneously distracting the medial and lateral compartments undera preset load (equal or unequal).

Calculate by the computer predicted gap data based on an initialplacement of the femoral and tibial implants and display predicted gapdata on user interface, this initial placement may be based at least inpart from user preference data for implant positioning, and/or from gapdata (for example equal implant gaps in extension and flexion, andsymmetric gaps from medial to lateral).

Adjust the position of the femur and/or tibial implants on the bones andrecalculate the change in the predicted gaps based on the adjustedposition

Make resections according to the final plan, insert femoral componentsand tibial baseplate.

Assemble ligament balancer with the corresponding tibial baseplate andaugments that match the tibial insert to be inserted in the knee thatwill articulate with the femoral components to be implanted, reinsert inthe knee joint (fixing to tibia as required).

Set ligament balancer to height of corresponding tibial insert to beimplanted (for example 10 mm) and assess final balance and soft-tissuetension using force readings and kinematic knee data at various flexionangles using ligament balancer as virtual trial.

In the above described mode, the distractor may be tilted to accommodatevarying degrees of joint line tilt due to the tibial and femoral anatomywhen it is inserted prior to any resections being performed.Alternatively, the distractor could have a degree of adjustability inthe height between the medial and lateral lower paddles to accommodatethe different heights of the tibial plateaus.

Several variations to the present invention can be envisioned. The upperarms may have surfaces adapted for articulating with the femoralcomponent directly so that augments do not need to be attached. Themedial and lateral gaps can be the heights reported by the ligamentbalancer instead of the heights measured the 3D position trackingsystem. The system can be configured to operate in a stand-alone modethat doesn't require a 3D tracking system. Accelerometers and/orgyroscopes can be built directly into the ligament balancer to measureits position. The controllers can be wirelessly connected to a displayor tablet computer with touchscreen that displays the user interface.The ligament balancer could have only one upper paddle and onemotor/gear/slider assembly instead of two. In the case of total knee,uni-knee, and other arthroplasty procedures, the ligament balancer maybe mechanically coupled to cutting or drilling guides, to optimallyposition a cutting guide at the appropriate resection level to havebalanced ligament tension. For instance, a cutting guide (or drill guidefor drilling holes in the bone for receiving pins for securing a cuttingguide) can be attached to the upper paddles, the lower paddle or thebody of the ligament balancer (which is in a fixed position relative tothe lower paddle). When the ligament balancer is inserted in the kneeafter a tibial cut is made, and the uncut femur is positioned withrespect to the tibia under the desired tension created by the ligamentbalancer in force or height control mode, the holes can be directlydrilled in the femur using the guide attached to the lower paddles orbody. This marks the location of the cutting guide such that theappropriate amount of bone is resected to replicate the desired tension.This can be performed for a distal femoral and/or a posterior femoralresection, for example, in total or uni-compartmental knee arthroplasty.

In accordance with an aspect, the computer aided orthopedic surgerysystem of the present invention includes a ligament balancing userinterface, as shown e.g., in FIG. 17A, an implant planning userinterface, as shown e.g., in FIG. 17B, and a post-operative kinematicsuser interface, as shown e.g., in FIG. 17C.

In accordance with another preferred embodiment, the present inventionprovides a kit 600, as shown in FIGS. 21 and 16. The kit 600 includes aplurality of femoral trail implants 602 of incrementally different sizesand an orthopedic distraction device, such as ligament balancer 1. Theorthopedic distraction device 1 can as described in any of the aboveembodiments and includes a first upper paddle 21, a plurality of lowerpaddles 12′, and a displacement mechanism 9 having a drive assemblyoperable to move the upper paddle relative to the lower paddle. Eachlower paddle 12 is independently connectable to the displacementmechanism. The orthopedic distraction device further includes aplurality of augments 43 each releasably connectable to the first upperpaddle. Each of the plurality of augments 43 has an articulating surfacethat corresponds in size to a size of each of the plurality of femoraltrial implants 602.

The kit 600 further includes a plurality of tibial implants 604. Each ofthe plurality of lower paddles 12′ has an overall profile sized andshaped to correspond to a size and shape of an overall profile of theplurality of tibial implants. A plurality of tibial insert implants 606can optionally be included in the kit 600.

The present invention as described in the above embodimentsadvantageously reduce the number of instruments (manual trials) in theOR. Typically, a range of tibial trial (or provisional) baseplates,tibial trial inserts and femoral trails are made available in theoperating room to allow a surgeon to provisionally insert into the jointand trial the size of prosthesis to be implanted in the joint. Trialingallows the surgeon to be sure that the selected implant size is thecorrect fit and provides the patient with the correct soft-tissuetension and balance, before opening and inserting the final implantcomponents into the joint. Typically, the range of sizes offered for thetibial baseplate and femoral component can be anywhere from 6 to 12 perimplant (tibia and femur). Moreover, each size of tibial insert implantcan be offered in several different thicknesses, for example 7 differentthicknesses may be offered: 10 mm, 11 mm, 12 mm, 14 mm, 16 mm, 18 mm and20 mm. When combining this with the number of different sizes of tibialbaseplates and femoral components, this can lead to a large number oftibial insert sizes that need to be included in the instrument set (forinstance 6×7, or 42 different tibial insert sizes and thicknesses).Moreover, if different styles of tibial inserts are offered (forexample, cruciate retaining (CR), ultra-congruent (UC), or posteriorstabilized (PS)), one per type may also need to be provided. However,providing every size of implant as a trial component in the operatingroom can lead to increased costs and complexity due to the large numberof components that need to be manufactured by the implant company, andwhen used on a reusable basis, cleaned and re-sterilized by the hospitalafter every case. Moreover, having a large amount of instruments in theOR makes the procedure more complex with more parts to deal with andmore space taken up with instrumentation on the back table of the OR. Anobject of the present invention is to provide a system that reduces thenumber of instruments that are required for trialing in the operatingroom. Additionally, conventional trial implants do not provideinformation as to the forces acting on the joint during the procedure,which can affect the outcome of the surgery. Thus the present inventionprovide an improved trialing process that provides feedback to surgeonas to the forces acting on and being applied to the joint.

It will be appreciated rotational by those skilled in the art thatchanges could be made to the preferred embodiments described abovewithout departing from the broad inventive concept thereof. It is to beunderstood, therefore, that this invention is not limited to theparticular embodiments disclosed, but it is intended to covermodifications within the spirit and scope of the present invention asdefined by the claims.

We claim:
 1. An orthopedic distraction device comprising: an upperpaddle for engaging a first bone of a joint; a lower paddle for engaginga second bone of the joint; a displacement mechanism operable todisplace the upper paddle relative to the lower paddle; and a controlleroperatively in communication with the displacement mechanism, andconfigured to apply varying displacement forces to displace the upperpaddle from the lower paddle based on a relative position between thefirst and second bones of the joint.
 2. The orthopedic distractiondevice of claim 1, wherein the displacement mechanism includes a driveassembly to displace the upper paddle relative to the lower paddle. 3.The orthopedic distraction device of claim 1, wherein the controller isconfigured to apply said varying displacement forces throughout a rangeof motion of the joint.
 4. The orthopedic distraction device of claim 1,wherein the relative position between the first and second bones definea joint angle of the joint.
 5. The orthopedic distraction device ofclaim 1, wherein the relative position between the first and secondbones define a flexion angle.
 6. The orthopedic distraction device ofclaim 1, wherein the controller comprises a memory having stored thereona predetermined force profile for applying said varying displacementforces.
 7. The orthopedic distraction device of claim 1, wherein theforce versus flexion angle profile is defined by a user and stored inthe memory for determining the varying displacement forces to apply. 8.The orthopedic distraction device of claim 1, wherein the force versusflexion angle profile for determining the varying displacement forces toapply is adjustable by a user intraoperatively during surgery.
 9. Theorthopedic distraction device of claim 1, wherein the varyingdisplacement forces are adjustable.
 10. The orthopedic distractiondevice of claim 1, wherein the force versus flexion angle profile fordetermining the varying displacement forces to apply is adjustable by auser throughout a range of motion of the joint.
 11. The orthopedicdistraction device of claim 1, wherein the force versus flexion angleprofile is displayed on a display.
 12. The orthopedic distraction deviceof claim 11, wherein the force versus flexion angle profile on thedisplay is adjustable by a user.
 13. The orthopedic distraction deviceof claim 11, wherein the force versus flexion angle profile on thedisplay includes node control points adjustable by a user.
 14. Theorthopedic distraction device of claim 1, wherein the controller isfurther configured to measure a gap spacing between the first and secondbones of the joint upon applying said varying displacement forces anddetermine an implant position based off the measured gap spacing. 15.The orthopedic distraction device of claim 14, wherein the controller isconfigured to measure the gap spacing while the first and second bonesof the joint are at about 0 or 90 degrees flexion.
 16. The orthopedicdistraction device of claim 1, wherein the controller is furtherconfigured to measure gap spacings between the first and second bones ofthe joint upon applying said varying displacement forces while therelative position of the first and second bones of the joint aresubstantially in an extension position, a mid-flexion position, and asubstantially 90 degree flexion position, and determine an implantposition based off the measured gap spacings at the extension position,the mid-flexion position, and the substantially 90 degree flexionposition.
 17. The orthopedic distraction device of claim 1, wherein thecontroller is further configured to measure a gap spacing between thefirst and second bones of the joint upon applying said varyingdisplacement forces while the relative position of the first and secondbones of the joint are at full extension, deep flexion or mid-flexion,and determine an implant position based off the measured gap spacing atfull extension, deep flexion or mid-flexion.
 18. The orthopedicdistraction device of claim 1, wherein the controller is furtherconfigured to determine a predicted alignment between the first andsecond bones of the joint upon applying said varying displacementforces, and determine an implant position based on the predictedalignment.
 19. The orthopedic distraction device of claim 18, whereinthe predicted alignment is based on a mechanical axis of one of thefirst and second bones and a sagittal plane of the other of the firstand second bones.
 20. The orthopedic distraction device of claim 1,further comprising a three-dimensional position tracking system fortracking the relative position between the first and second bones of thejoint.